message for plugin errors does not include how to fix them

[Leo Arias]

When trying to make a snap, but there's a plugin error, the same exception is raised for all the possible errors:

- unknown plugin
- no plugin found in module
- properties failed to load

This exception only mentions the problem, but doesn't explain how to fix it. To add this information, we need to either split the exception, one for each case, or add a field for the information about the fix.

[Kyle Fazzari (kyrofa)]

Yeah these could be split. The resolution is probably to assume they made a typo.

[Carlo Lobrano (c-lobrano)]

I had a look into the code and it seems that this is fixed already (a PluginError message that differentiate between the three cases is present), or I didn't understand the issue :)