Distinguish between name not provided and incorrect

When loading from config we need a way to determine if a plugin name was
specified incorrectly or was not specified at all. We need this to
determine if we need to load a fallback plugin.

This is much more in line with how CLI loading works and how it should
have worked initially.

Change-Id: I5547b6e169abc4f1850ff205a8f054a617785c2c
Closes-Bug: #1359618