Better error messaging for parser errors

[stanistan]

We can have the parser actually throw a diagnostic error instead of just being like "yo this bad". We'll have to wait for a new version of Ohm since ohmjs/ohm#218 was merged, the lines to changes are here:

parser-js/index.js

Lines 10 to 11 in 580d966

	// TODO use `matchResult.message` or `matchResult.shortMessage`
	// once https://github.com/harc/ohm/pull/218 is resolved