fa6ab8ada1 rpc: Return more specific reject reason for submitblock (MarcoFalke)
Pull request description:
The second commit in #13439 made the `TODO` in the first commit impossible to solve. The meaning of `fNewBlock` changed from "This is the first time we process this block" to "We are about to write the new *valid* block". So whenever `fNewBlock` is true, the block was valid. And whenever the `fNewBlock` is false, the block is either valid or invalid. If it was valid and not new, we know it is a `"duplicate"`. In all other cases, the `BIP22ValidationResult()` will return the reason why it is invalid.
Backport Core PR13983
Depends on D4046