7264.bugfix

Fixed a case where custom plugins specified by command line could silently fail.

Specifically, if a plugin relies on the ``init-hook`` option changing ``sys.path`` before
it can be imported, this will now emit a ``bad-plugin-value`` message. Before this
change, it would silently fail to register the plugin for use, but would load
any configuration, which could have unintended effects.

Fixes part of #7264.