Fix return error when resource can't be found

By current implement, the HTTP request for /v2/resources/<resource_id>
will return 500 if there is no coresponding resource or the resource id
is invalid. And the response is as below:

{"error_message": "{"debuginfo": null, "faultcode": "Server",
"faultstring": "list index out of range"}"}

After fixed, the response will be like this:

{"error_message": "{"debuginfo": null, "faultcode": "Client",
"faultstring": "Invalid input for field/attribute resource_id.
Value: '9'. Unknown resource"}"}

Fixes bug: 1195925

Change-Id: I73c73fb3ba57bc6cbbf421f9ac5ec3cdb68ce784
Merge "Fix return error when resource can't be found"