Better error handling

This is a good project but it's very hard to know what's wrong. We only know when there is a problem by getting undefined.. But if the expression does not return anything and only have a side effect then we don't even know if the code was executed or if there was a problem in the AST.

I suggest `static-eval` could use `Errors` instead of the FAIL object. This way it would not be needed to check everytime of something is equal fo `FAIL` and the end user would get more information about the error.

For example I spent 30 minutes figuring out why I was receiving `undefined` for a perfectly valid expression. It was simply that 'NewExpression' was not handled (This is not a problem of course, this is open source community someone else can implement that) but with Errors I would have known right away.

Would you accept a pull request using `Error` instead of `FAIL` ?