Improve Constant Contact error handling

[miguelpeixe]

The ESP methods throw exceptions. These should be replaced with WP_Error() as it's the expected error handling already used for other providers.

[leogermani]

Maybe related to that, if you save CC as the provider without credentials, the site throws errors and the settings page becomes unusable

[leogermani]

Also, we could have only one instance of the SDK in the main provider class instead of creating an instance in every method