I got tired of having to hack in a panic hook bypass whenever I wanted a
backtrace with line numbers. Now a dev channel build will behave more
like the default panic hook, printing a pretty traditional backtrace
message and exit with an error code instead of aborting to avoid the
annoying "Zed crashed" dialog.

I have plans to modify our panic reporting to be able to have line
numbers reported without breaking the de-duping but I haven't done that
yet.

Additionally I slightly improved what we do in threads which panic as a
result of another thread's panic.

Release Notes:

- N/A