Provide better error on `@client` annotation in `MockLink` and missing local resolvers

[SimenB]

See discussion in apollographql/react-apollo#3316.

The current error provided by MockLink is just No more mocked responses for the query: followed by a dump of the query and arguments provided.

While providing mockResponses automatically strips out @client fields (apollographql/react-apollo#2776), there's no such happening for requests. This leads to a situation that is hard to debug, since the seamlessly removed part from one place is the thing that leads to an obfuscated error in another place.

I don't know what the ideal solution is, but a few suggestions of varying quality/usefulness:

1. Try to diff all provided responses with the query, instead of requiring the stringified value to be identical, and print a nice diff which will at least highlight why there's "no more mocked responses". Landing Improve mismatched query error message react-apollo#2883 might be enough here? Not sure
2. Throw an explicit error if a query contains a @client field which has no associated local resolver provided
3. A simpler version of the above is to throw if there are @client fields without any resolvers being passed without trying to match them up
4. provide a way strip out @client fields from the query (similar to what happens automatically for responses)
5. Instead of automatically stripping out @client fields from responses, throw an error and require the user to explicitly opt in to it

[jeremy93-2008]

When it-s will be available, actually I just mostly spending the whole day figuring what-s going on, when it-s just some black box stuff problem.... the feature was open in 2019, come on, I-m sure that we can add a little warning :)