General improvements and fixes found during testing

[devbugging]

This PR includes multiple improvements for issues found during testing, some are:

• Improve general API error handling to return null instead of not found errors as the clients expect such behaviour,
• track nonce with block height so if a nonce is updated for same block height is not incremented
• Improved logging and error descriptions

[m-Peter]

I guess we should return some kind of error here, instead of both values being nil, nil.

[m-Peter]

Nice one 👍 I think the evm.TransactionExecuted event payload is going to contain BlockHash, so we can avoid the extra DB call.

[m-Peter]

Very nice 👏
Left only a few comments for discussion.