In parse_number_string, if there is a parse error, report the specific
error as one of the values in a new parse_error enum, and update
lastmatch to match the error location. This allows users of the library
to print more helpful error messages for invalid inputs.
Revert #251, and instead allow an invalid trailing empty exponent to be
treated as junk data in JSON parsing. Expand the test suite to test this
case, including testing the trailing junk.
