Wait for event after registerNonFinal

[matthiasgeihs]

Location: backend/ethereum/channel/register.go

Context: Previously, after calling registerNonFinal, we waited for the corresponding Registered event on the contract. This was changed at some point due to the complex subscription logic involved.

Problem: Currently, we don't wait for the event, and we also do not check that the transaction is correctly mined. We only log a warning. This means that the function call does not return an error if the transaction failed.

Potential fix: As a quick fix, we should return an error if the transaction failed. As a comprehensive fix, we should use again a subscription that checks that corresponding events are emitted.