Error handling

[bojanz]

We need a way to recognize whether a Composer process completed successfully or not. Users ignoring the Composer output is a real problem. If we can detect an error, we can probably also extract the error message, and show it.

[mglaman]

There is ex, stdout, and stderr, right now all if piped out. This needs to be groomed a lot. Like moving it out of a textarea so we can do HTML highlighting.