cider-ns-refresh: summarize errors as an overlay

Most times, code reloading errors come from a 'silly' mistake that doesn't deserve much stacktrace inspection.

So, in addition to the displayed stacktrace, we could present the gist of the problem as a red overlay, similar to what we've done in recent times.

While we're there, we could add defcustoms controlling both widgets (overlay, stacktrace)