Fix p* scripts to produce an error when given a bad config_uri

[kpinc]

When a bad settings URI (usually a path to a non-existant file) is
supplied to one of pyramid's command line programs, and in general
when there is a reason why a settings file cannot be used, produce
an error message instead of a traceback.

Applications which import pyramid.paster must handle the
plaster.exceptions.PlasterError exception themselves.

[kpinc]

I have changed the github workflow to get this to pass CI. This change is unrelated to the purpose of the patch. (macos-11 is no longer offered.)

[kpinc]

To see what this fixes, try:pserve /tmp/nonexistantfile
You get a traceback without this patch.