Enhancement: Allow parsing one rule to fail cleanly

[Northern-Lights]

When using func (p *Parser) Parse(text string), a rule that does not match the grammar exits the program with status 1. This is an example:

{"lexical":"error","line":"4","msg":"Line 4: illegal regex modifier ())"}exit status 1

It would be great if the method had this signature instead:

func (p *Parser) Parse(text string) error

If we could pass errors from the lexer up to this method, maybe we can ignore rules that do not compile without exiting the application.

[Xumeiquer]

Yeah, that's true. This is a feature I have in mind.