Separate exceptions for authorization and permission errors (401 and 403 response codes).

[kalnik-a-a]

These changes will simplify verification of API keys and their access permissions without necessity of parsing SparkPostException message.

[yepher]

@kalnik-a-a,

Thanks or the pull request. Yes, absolutely those specific exception types will make the library much easier to use. This will be added to the v0.15 release that is currently in progress.

[kalnik-a-a]

@yepher

Great, if similar changes are already planned or done feel free to close this pull request.
Btw, can you please tell aproximate date of v0.15 release?

[yepher]

I am looking to release by Friday end of day April 8th.

I was not already adding specific Exception types so this is perfect. The other planned changes were ip_pools support and concrete types for retrieving templates.