[draft] Add error codes to resource server

[mkurapov]

Context

Similar to how we have in the auth server, we should add error code enums to the resource server.

The initial list of error.codes should be:

• invalid_client (to be used for invalid keys & signatures)
• insufficient_grant
• invalid_token
• inactive_token
• invalid_request (syntax error, not found, amount mismatch)
• destination_unreachable WIP (for quote errors)
• grant_limits_exceeded ? (or merge with insufficient_grant)