snapcraft: do not suppress ImportErrors for top-level exceptions with local plugins

[Nish Aravamudan]

Currently, snapcraft suppresses all ImportErrors when attempting to import local plugins (possibly all plugins?) This might be sensible for deep ImportError exceptions, but when first coming up to speed, particularly with local plugins, it's quite common to use the wrong name (as the x-plugin.py format is counter-intuitive) and the inheritance for some of the plugin classes has been finicky in the past. I think it might make sense to not suppress such exceptions if they are happening in shallow stacks (e.g. top-level exceptions).

I hit this when I had a local plugin that was importing the wrong file name. And then was using the wrong parent plugin name. But both get masked by 'unknown plugin' during cleanbuild.