Improve operation error handling

[Desuuuu]

Currently, operation errors are not treated as protocol errors due to the lack of error code. They look like this:

{"errors":[{"message":"operation badOp not found"}],"data":null}

Or, if no operation is provided (e.g. the query is empty):

{"errors":[{"message":"operation  not found"}],"data":null}

This change aims to improve on this by:

• Adding the GRAPHQL_VALIDATION_FAILED error code when an invalid operation name is provided, making transports treat it as a protocol error.
• Returning a separate error when no operation is provided.

This was previously requested here: #1786

I also added 2 tests covering the changes to the executor.

[StevenACoffman]

Thanks! This is great! A lot of folks have struggled with less than helpful error messages in a variety of situations, so I really appreciate efforts to make it easier to understand when things go wrong.

[coveralls]

Coverage increased (+0.05%) to 74.993% when pulling 6d5fcc2 on Desuuuu:operation-errors into 2526f68 on 99designs:master.