need better error messages

[dt-rush]

Currently, failing during parsing just leads to something like:

Error: match failed
    at .../node_modules/ometa-js/lib/ometajs/core.js:64:15
    at fail (.../node_modules/ometa-js/lib/ometajs/core.js:11:20)
    at Object.<anonymous> (.../node_modules/ometa-js/lib/ometajs/core.js:16:2)
    at Module._compile (module.js:653:30)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Module.require (module.js:597:17)
    at require (internal/module.js:11:18)

And if you look in ometajs/core.js, those lines are nothing of interest, those are just the definitions of the fail() function, basically. They don't even have access to the token that we were at when we failed.