added UnexpectedToken to __init__ for help unit testing

lark/__init__.py

@@ -1,5 +1,5 @@
 from .tree import Tree, Transformer, InlineTransformer
-from .common import ParseError, GrammarError
+from .common import ParseError, GrammarError, UnexpectedToken
 from .lexer import UnexpectedInput, LexError
 from .lark import Lark
 from .utils import inline_args