When the rails addon crashes on startup there's no indication of where the error happened

If the rails addon crashes on startup, there's no indication of where the problem is. There's the long logs in the "Output" tab, but you'd have to know to look there, and they can be rather large especially with the long backtraces common with rails applications.

I coded up a simple POC to parse the exception text for a meaningful message, and last source location under the workspace, and send a Diagnostic: #649 so that the problem is revealed in the "Problems" tab under the editor. It's not necessarily pretty, and might not handle all situations where there's a runtime error, and it doesn't seem like it'll result in the rails addon being retried once it fails once (even if the source of the problem is fixed). But, it might provide a good jumping off point for someone more familiar with this project to take on.

Start by reviewing the diagnostic POC described in issue #649 and the Rails add-on's startup error path. Compare the exception text and workspace source-location handling with the existing Output logs. Done means startup failures expose a useful message and source location in the editor's Problems tab.