Add opam error to unclutter output in case of package errors

[AltGr]

and easily browse/report the logs. That would be better than the current middle-ground with error messages a bit too long, including pointers to several different log files, and a bit of the output that doesn't always include the relevant information.

The stdout/stderr snipped could be kept, but removing all the logs and just advertising opam error (maybe just once at the end: there were some errors, run "opam error" to see the logs) would be nicer.

See #2111 for the origin of the idea.