Fix plugin_loader refactor gone wrong

[cydanil]

Description

During refactoring, self.errors was changed to a set, when it's actually a dictionary.
This issue arises when plugins are not correctly set up.

How Has This Been Tested?

This was tested by launching the application without any of the plugins being available.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.