Less panics, more Results

[Robbepop]

Currently Prophet handles too many (potential) errors with panics instead of proper Rust-style error handling via Result return type and proper error kinds etc. This also leads to less testable code and prevents implementing better error reporting strategies and should be addressed.

Branch next already implements (decent) error kinds and a rust-style Error struct with typical interface impls. The next step is to adjust the rest of the code base to this format away from asserts.