Return all errors in TransportQueryError exception

[leszekhanusz]

• Add an errors attribute in the TransportQueryError to get all the errors in the exception
• expose private _execute and _subscribe functions to get ExecutionResult data directly if needed

[coveralls]

Coverage increased (+0.01%) to 99.225% when pulling a5313e0 on leszekhanusz:feature_return_all_errors_in_transport_query_error_exception into a21ff73 on graphql-python:master.

[coveralls]

Coverage increased (+0.01%) to 99.224% when pulling 9a22931 on leszekhanusz:feature_return_all_errors_in_transport_query_error_exception into ce13236 on graphql-python:master.

[leszekhanusz]

Note: instead of private _execute and _subscribe functions, we could have public functions with a more explicit name (something like execute_execution_result and subscribe_execution_result but it is a little too much verbose for my taste)