Write panic error and stacktrace to the apps logger. Panics that occur
while the ui is running leave the terminal in a garbled state, making it
difficult to read the stacktrace. The app log is the logical place to
Panic stack traces are dumped to stderr.
I don't think that redirecting it to the log is a good idea, considering that
most people just run `aerc` and hence /dev/null the log.
Just keep it in stderr, so if people do notice a crash they can simply redirect
stderr to somewhere else than the current terminal.