Error propagation, annotation and handling

[janos]

Consolidate error handling and improve error messages. Errors should be constructed so that it is safe to expose them to the user revealing as less as possible implementation details.

• Errors must be propagated by the caller function
• Error should not be logged and passed at the same time
• It is up to the next caller function to decide what should be done in case of an error
• If a function has multiple returns with, any returned error should be annotated either with fmt.Errorf using %w verb or with custom error type that has Unwrap method