Uninstall: tolerate hook failures when force=true (#16513)

Fixes #16478

This allows an uninstall to proceed even when encountering pre-uninstall
hook failures if the user chooses the --force option for the uninstall.

This also prevents post-uninstall hook failures from raising an exception,
which would terminate a sequence of uninstalls. This isn't likely essential
for #16478, but I think overall it will improve the user experience: if
the post-uninstall hook fails, there isn't much point in terminating a
sequence of spec uninstalls because at the point where the post-uninstall
hook is run, the spec has already been removed from the database (so it
will never have another chance to run).

Notes:

* When doing spack uninstall -a, certain pre/post-uninstall hooks aren't
  important to run, but this isn't easy to track with the current model.
  For example: if you are uninstalling a package and its extension, you
  do not have to do the activation check for the extension.
* This doesn't handle the uninstallation of specs that are not in the DB,
  so it may leave "dangling" specs in the installation prefix