Proper error handling with explanatory error messages

[gahjelle]

Add custom exceptions, and error messages with instructions on how to fix or investigate:

• [x] When importing a package that does not exist
• [x] When importing a module that does not exist
• [x] When looking for a plugin function that does not exist
• [x] When trying to use a module as plugin where no functions are registered
• [x] When calling a plugin with the wrong arguments