Unify runtime error types

[svyatonik]

may be fixed separately or as a part of #2457

We are using pallet-level errors, but there are errors that appear somewhere inside the "glue" runtime code and they are then logged (logs are unavailable for release runtimes) and are then merged into some "big" pallet-level error. E.g. when messages pallet is verifying messages proof, there's a lot of bad stuff that can happen there - e.g. invalid messages range, unknown header, invalid storage root, duplicate items in the storage proof, unused items in the storage proof, ... But in the end the transaction will fail with the InvalidMessagesProof error, which isn't super-informative.

Let's make either bridges-level error enum or pallet-level enums of what can happen during the call and use it (along with frame_support::PalletError - cheers to @serban300 for finding this) instead of &' static str and other custom errors.