Build Exception handling

[smashwilson]

When a build causes an internal DCIY exception, right now it fills the build output with a giant stack trace:

A better idea is to catch any thrown exceptions, store them on the Build object, and show them below. We can have "friendly text" for the expected ones (like BranchNotFoundError) and show the full trace in an initially-collapsed <div>.

Relocated from a discussion on #33.

[cobyism]

👍 It would be great if there was a way to help people submit issues on GitHub when they encounter exceptions in DCIY itself, too. Don’t know how that might be done, but worth thinking about.