Actual exceptions on requests to Mandrill server are not propagated

[thberger]

The execute() method of MandrillRequestDispatcher has a bug: If the request to Mandrill fails for some reason, the response object gets null. This in turn leads to a NPE in the finally block. When the method is finally left with the NPE, the original exception cause is gone - making debugging very hard.
This codesmell is also detected by SonarLint, by the way.

PR is incoming.

See also: https://next.sonarqube.com/sonarqube/coding_rules#rule_key=csharpsquid%3AS1163