Better error handling

[KyrietS]

Many functions in this project simply return Result<()>. Sometimes it's Result<Something, String> to pass some text error message. There are no custom Result classes whatsoever. A complete redesign of error handling is necessary to proceed with new features.

Key points for new error handling system:

• generalized - there should be one major error type
• extensible - it should be easy to add new error types
• descriptive - error should be able to contain string message, maybe some error code (but not necessarily)

[KyrietS]

Idea: instead of Result<Something, String> it's better to do Result<Something, MyErrorEnumType> where MyErrorEnumType can also store a string if it's necessary. I could not only print every error as always but also match them.