[Enhancement] Improve error handling in ET_Client instantiation

[mourtisma]

Is your feature request related to a problem? Please describe

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
2. Potential HTTP errors coming from requests are not properly handled
3. In the ET_Client params, applicationType, if not None or blank, can be anything

I already created a PR to improve those 3 points: #123

Solutions

1. Create a base class for exceptions raised by the ET_Client, and subclasses for the different scenarios (configuration, loading WSDL, token refresh)
2. Use Response.raise_for_status to handle HTTP error responses coming from requests
3. 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