Stop printing stacktraces in pmbootstrap log when not necessary

Right now, pmbootstrap prints a stacktrace to pmbootstrap log whenever it fails, even if the failure is something where the stacktrace is not at all relevant, e.g. when a package fails to build. This adds noise and is most of the time not useful. Therefore, I propose that we by default hide the stacktrace, and enable it via something like an environment variable. Maybe something like if you set PMB_DEBUG or PMBOOTSTRAP_DEBUG.

Does anyone have any hard feelings about changing the behaviour to this?