bug#6088: Please record all warnings and errors in the *messages* buffer

[Lennart Borgman]

Could we please coordinate warnings, errors and messages? This could
make it more easy to find errors.

I suggest as a simple first step to record all errors and warnings in
the message buffer.

For warnings this is very simple. Just move the code for
noninteractive in display-warnings so that it is always run.

For errors this means outputting a message even when debug-on-error is
non-nil. I am not sure how to do that.