Clean up all error handling, remove panics

[lthoerner]

There seems to be little need for dynamic error handling in the project, and there are many areas in the code which still use .unwrap() and other panic-prone
code. This PR aims to fix both issues and provide a more easy-to-understand error API.

[lthoerner]

This is mostly done - @doinkythederp will need to remove panic-prone calls in the plugins module when working on its implementation further, but I felt that it would be more relevant to a plugin feature PR.