ParserKeyword: make the exception messages more verbose

mostly, include the keyword names, so that the user can fix these
errors without using gdb. (Not that I mind gdb, but I have been told
that gdb is a big no-no. ;)

(this patch also fixes a few typos...)
Merge pull request #344 from andlaus/more_useful_exception_messages

ParserKeyword: make the exception messages more verbose