Improved error message when using promise keyword inapropriately

[adiel]

Sometimes I've used the name promise for a variable forgetting it's a keyword, e.g:

promise = getStuffAsync(...)

//later
promise!

The pogo compiler says "TypeError: Cannot read property '0' of undefined" with no trace of where the problem is. Takes me a while to remember what this means.

[refractalize]

good point!