Fix misleading "Method not found" multiwallet errors

Summary:
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.

Fixes T185

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Maniphest Tasks: T185

Differential Revision: https://reviews.bitcoinabc.org/D1024