Refine INVALID_SIGNATURE to RECEIVER_SIG_REQUIRED when appropriate

[tinker-michaelj]

Summary
It would be nice to refine the INVALID_SIGNATURE response code, as it can mean multiple things.

In particular, when the key on an account X with receiverSigRequired=true did not supply a valid signature, we could return RECEIVER_SIG_REQUIRED rather than INVALID_SIGNATURE.

[gregscullard]

Agreed, was thinking the same myself only a few days ago.