Fix misleading "Method not found" multiwallet errors
Raise RPC_WALLET_NOT_SPECIFIED instead of RPC_METHOD_NOT_FOUND when a required
wallet filename was not specified in an RPC call.
Also raise more specific RPC_WALLET_NOT_FOUND error instead of
RPC_INVALID_PARAMETER in case an invalid wallet was specified, for consistency.
Change wallet method disabled error text
Not strictly backwards compatible because the error is not new in this release.
Test Plan: make check
Reviewers: deadalnix, schancel, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Maniphest Tasks: T185
Differential Revision: https://reviews.bitcoinabc.org/D1024