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

Sam Thursfieldrequested to merge
sam/plugin-error into master

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'

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