Changed the "ERROR" token in the grammar to the default "error", since
it was causing exceptions and this allows the tree to be generated up to
a certain point without crashing.
Forgot a word in the comment for the ERROR terminal in the CUP file.
Edited the lexer to explicitly store the boolean value of the "true" and
"false" keywords to avoid storing them as strings and causing exceptions
in the parser.
Fixed some wording in a comment again.