diff --git a/lark/lexer.py b/lark/lexer.py
index b568e53..db58bb2 100644
--- a/lark/lexer.py
+++ b/lark/lexer.py
@@ -7,6 +7,17 @@ from .utils import Str
 class LexError(Exception):
     pass
 
+class UnexpectedInput(LexError):
+    def __init__(self, seq, lex_pos, line, column):
+        context = seq[lex_pos:lex_pos+5]
+        message = "No token defined for: '%s' in %r at line %d" % (seq[lex_pos], context, line)
+
+        super(LexError, self).__init__(message)
+
+        self.line = line
+        self.column = column
+        self.context = context
+
 class Token(Str):
     def __new__(cls, type, value, pos_in_stream=None):
         inst = Str.__new__(cls, value)
@@ -103,8 +114,7 @@ class Lexer(object):
                     break
             else:
                 if lex_pos < len(stream):
-                    context = stream[lex_pos:lex_pos+5]
-                    raise LexError("No token defined for: '%s' in %s at line %d" % (stream[lex_pos], context, line))
+                    raise UnexpectedInput(stream, lex_pos, line, lex_pos - col_start_pos)
                 break
 
 
diff --git a/lark/load_grammar.py b/lark/load_grammar.py
index d090cc7..e7d1bf4 100644
--- a/lark/load_grammar.py
+++ b/lark/load_grammar.py
@@ -1,10 +1,11 @@
 import re
 import codecs
 
-from .lexer import Lexer, Token
+from .lexer import Lexer, Token, UnexpectedInput
 
 from .parse_tree_builder import ParseTreeBuilder
 from .parser_frontends import LALR
+from .parsers.lalr_parser import UnexpectedToken
 from .common import is_terminal, GrammarError
 
 from .tree import Tree as T, Transformer, InlineTransformer, Visitor
@@ -285,9 +286,19 @@ class GrammarLoader:
         self.rule_tree_to_text = RuleTreeToText()
 
     def load_grammar(self, grammar_text):
-
-        token_stream = list(self.lexer.lex(grammar_text+"\n"))
-        tree = self.simplify_tree.transform( self.parser.parse(token_stream) )
+        try:
+            token_stream = list(self.lexer.lex(grammar_text+"\n"))
+        except UnexpectedInput as e:
+            raise GrammarError("Unexpected input %r at line %d column %d" % (e.context, e.line, e.column))
+
+        try:
+            tree = self.simplify_tree.transform( self.parser.parse(token_stream) )
+        except UnexpectedToken as e:
+            if '_COLON' in e.expected:
+                raise GrammarError("Missing colon at line %s column %s" % (e.line, e.column))
+            elif 'tokenvalue' in e.expected:
+                raise GrammarError("Expecting a value at line %s column %s" % (e.line, e.column))
+            raise
 
         # =================
         #  Process Tokens
diff --git a/lark/parse_tree_builder.py b/lark/parse_tree_builder.py
index ee59f71..5561544 100644
--- a/lark/parse_tree_builder.py
+++ b/lark/parse_tree_builder.py
@@ -1,4 +1,4 @@
-from .common import is_terminal
+from .common import is_terminal, GrammarError
 
 class Callback(object):
     pass
@@ -70,7 +70,8 @@ class ParseTreeBuilder:
 
                 alias_handler = create_rule_handler(expansion, f)
 
-                assert not hasattr(callback, _alias)
+                if hasattr(callback, _alias):
+                    raise GrammarError("Rule expansion '%s' already exists in rule %s" % (' '.join(expansion), origin))
                 setattr(callback, _alias, alias_handler)
 
                 new_rules.append(( _origin, expansion, _alias ))
diff --git a/lark/parsers/lalr_parser.py b/lark/parsers/lalr_parser.py
index 2827925..d766221 100644
--- a/lark/parsers/lalr_parser.py
+++ b/lark/parsers/lalr_parser.py
@@ -1,6 +1,23 @@
 from .lalr_analysis import ACTION_SHIFT
 from ..common import ParseError
 
+class UnexpectedToken(ParseError):
+    def __init__(self, token, expected, seq, index):
+        self.token = token
+        self.expected = expected
+        self.line = getattr(token, 'line', '?')
+        self.column = getattr(token, 'column', '?')
+
+        context = ' '.join(['%r(%s)' % (t.value, t.type) for t in seq[index:index+5]])
+        message = ("Unexpected input %r at line %s, column %s.\n"
+                   "Expected: %s\n"
+                   "Context: %s" % (token.value, self.line, self.column, expected, context))
+
+        super(ParseError, self).__init__(message)
+
+
+
+
 class Parser(object):
     def __init__(self, ga, callback):
         self.ga = ga
@@ -20,18 +37,13 @@ class Parser(object):
                 return states_idx[state][key]
             except KeyError:
                 expected = states_idx[state].keys()
-                context = ' '.join(['%r(%s)' % (t.value, t.type) for t in seq[i:i+5]])
                 try:
                     token = seq[i]
                 except IndexError:
                     assert key == '$end'
                     token = seq[-1]
-                raise ParseError("Unexpected input %r at line %s, column %s.\n"
-                                 "Expected: %s\n"
-                                 "Context: %s" % (token.value, 
-                                        getattr(token, 'line', '?'),
-                                        getattr(token, 'column', '?'), 
-                                        expected, context))
+
+                raise UnexpectedToken(token, expected, seq, i)
 
         def reduce(rule):
             if rule.expansion: