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.