Reduce use of exceptions

[djcsdy]

The parser throws exceptions to signal that a grammar production cannot be matched and that the parser must backtrack.

This is a non-exceptional situation: we expect it to happen many times when parsing any XPath expression. The majority of parse exceptions thrown will be caught and handled. The only case where a parse exception will not be caught is if the parser has exhausted all backtracking possibilities without yielding a match; in other words, if the XPath expression itself is invalid.

It’s impolite to throw exceptions in such non-exceptional situations, and it may also be hurting parsing performance. We should consider an alternative mechanism to trigger backtracking. I’m not sure exactly what yet.

(It’s possible that haxe-xpath also throws and catches exceptions in some other non-exceptional circumstances, and of course those should be cleaned up too.)