_plugincontext.py: Fix unhandled exception if plugin is not found

This fixes an AttributeError triggered if we try to import a plugin from
a package which is installed but does not contain that plugin.

Before:

    [--:--:--] START   Resolving pipeline
    Traceback (most recent call last):
      ...
    AttributeError: 'NoneType' object has no attribute 'dist'

After:

    [--:--:--] START   Resolving pipeline
    [00:00:00] FAILURE Resolving pipeline
    Error loading pipeline: Pip package buildstream-external does not contain a plugin named 'nonexistant-plugin'