When validation of a transaction fails, informs the requester

Currently sending a transaction to a node is a fire-and-forget operation.
Even if the transaction is very obviously wrong (missing field, wrong signature / hash, etc...) the node just doesn't report anything.
This makes writing tests much more complicated than it should be.

Search the Go repository for the transaction submission and validation entry points, then trace how validation failures are currently handled. Define how a requester should receive errors for malformed transactions, invalid signatures or hashes, and add coverage for those failure cases. Done means transaction-sending tests can observe validation failures instead of treating submission as fire-and-forget.

Written by the indexing model from the issue text.