Improve error handling

[mourtisma]

Referenced Issue: #122

This PR tackles the following code improvements

1. When configuring the ET_Client, loading the WSDL and refreshing the token, the current code only raise base Exceptions, which are not ideal to handle for developers
   => Create a base class for exceptions raised by the ET_Client, and subclasses for the different scenarios (configuration, loading WSDL, token refresh)

2. Potential HTTP errors coming from requests are not properly handled
   => Use Response.raise_for_status to handle HTTP error responses coming from requests

3. In the ET_Client params, applicationType, if not None or blank, can be anything
   => Handle that case properly, and raise a new ConfigurationException in case of unallowed value

I also added tests for these error scenarios, in a specific file, FuelSDK/test_ET_Client_errors.py

[mourtisma]

@smunuswami @manivinesh

Hi everyone!

If you don't have time to look at this PR, feel free to ask someone else to do so.

I chose you two because you were my two first reviewer suggestions.

Thanks!

[mourtisma]

@smunuswami @manivinesh Hi everyone.
This PR has been opened for months, now.

Can someone please have a look (or suggest someone who could) at it ?
Thank you :)