[raft/scd] Return result and error in TransactWithResult

[MariemBaccari]

Chained PR: #1627 -> #1642 -> #1643 -> #1644 -> #1645 -> #1646 -> #1649 -> #1650 -> #1651 -> #1653 -> #1654 -> #1656 -> #1657 -> #1655 -> #1666 -> #1667 -> #1668 -> #1669

Fix TransactWithResult to return both the result and the error. This is needed by CreateOperationalIntentReference which returns both an error and a AirspaceConflictResponse result.

[mickmis]

Have you considered instead introducing a new type containing both return values (ChangeOperationalIntentReferenceResponse and AirspaceConflictResponse)?
Because:

• I'd say it's not very Go-idiomatic to return both a value and an error (although we already do that I agree)
• But worse: the handler (#1655) is going to have to have to cast the same result to different types and this is to be avoided if possible