rpc: fix invalid parameter error codes for {sign,verify}message RPCs

[barton2526]

RPCs that accept address parameters usually return the intended error code RPC_INVALID_ADDRESS_OR_KEY (-5) if a passed address is invalid. The two exceptions to the rule are signmessage and verifymessage, which return RPC_TYPE_ERROR (-3) in this case instead. Oddly enough verifymessage returns RPC_INVALID_ADDRESS_OR_KEY when the signature was malformed, where RPC_TYPE_ERROR would be more approriate.

This PR fixes these inaccuracies...

master branch:

$ ./bitcoin-cli signmessage invalid_addr message
error code: -3
error message:
Invalid address
$ ./bitcoin-cli verifymessage invalid_addr dummy_sig message
error code: -3
error message:
Invalid address
$ ./bitcoin-cli verifymessage 12c6DSiU4Rq3P4ZxziKxzrL5LmMBrzjrJX invalid_sig message
error code: -5
error message:
Malformed base64 encoding

PR branch:

$ ./bitcoin-cli signmessage invalid_addr message
error code: -5
error message:
Invalid address
$ ./bitcoin-cli verifymessage invalid_addr dummy_sig message
error code: -5
error message:
Invalid address
$ ./bitcoin-cli verifymessage 12c6DSiU4Rq3P4ZxziKxzrL5LmMBrzjrJX invalid_sig message
error code: -3
error message:
Malformed base64 encoding

Ref: bitcoin/bitcoin#18466

[jamescowens]

utACK