Feature/allow errors to be raised or returned

[alexmclarty]

PR to allow:

• errors to be raised with a custom exception (ResultError) which will allow better handling by consumers of the Client class.
• errors to be returned along with data which will allow consumers of the Client class to act on all errors rather than the first in a list.

I'd like to get some feedback on the approach before merging.

[leszekhanusz]

I though a lot about this proposal and now think that adding a raise_error flag in the client may not be our best option.

I made a new PR #96 with a proposed alternative.

• The TransportQueryError exception now returns all errors in an errors attribute.
• private functions _execute and _subscribe now exist in the sessions and can be used by the user if he wants to have results as ExecutionResult instances with errors instead of data and exceptions.