Give specifc error when service doesn’t exist

If you sign a token with a service ID that doesn’t exist (say, for
example, that you get service ID and API key mixed up) then you get
an error saying that “no API keys exist for the service”. This is wrong
because the service doesn’t even exist.

This commit adds:
- code to check if the service does exist
- a specific error message for this case

The check does mean an extra database call to look up the service.
However this only happens _after_ looping through all the API keys. So
it shouldn’t have a performance implication for anyone using a valid API
key.

app/authentication/auth.py

@@ -1,8 +1,11 @@
 from flask import request, jsonify, _request_ctx_stack, current_app
+from sqlalchemy.orm.exc import NoResultFound
+
 from notifications_python_client.authentication import decode_jwt_token, get_token_issuer
 from notifications_python_client.errors import TokenDecodeError, TokenExpiredError
 
 from app.dao.api_key_dao import get_model_api_keys
+from app.dao.services_dao import dao_fetch_service_by_id
 
 
 class AuthError(Exception):
@@ -48,6 +51,11 @@ def requires_auth():
         _request_ctx_stack.top.api_user = api_key
         return
 
+    try:
+        dao_fetch_service_by_id(client)
+    except NoResultFound:
+        raise AuthError("Invalid token: service not found", 403)
+
     if not api_keys:
         raise AuthError("Invalid token: no api keys for service", 403)
     else: