improve the error output (UX) #4
Description
Pligos currently is really rough on it's user. There are a lot of ways the program can actually panic when there is a mistake in the configuration. Pligos should respond with meaningful error messages, that help the user to configure the application correctly. E.g. the error message 'Context "foo" not found should result in Available contexts are "bar", "biz" and "baz".'
Also pligos could help the user to create a new config based on a flavor. But this should probably a different issue