Wrap CloudStack and Excon errors in ActionFailed

[tas50]

Bad API credentials currently surface as Fog::JSON::DecodeError with a stack trace, which tells the user nothing about what is wrong.

Confirmed by stubbing a 401 against create:

RAISED: Fog::JSON::DecodeError
is a Kitchen::Error? false

Test Kitchen prints non-Kitchen errors as a raw trace. Rescuing Fog::Errors::Error and Excon::Errors::Error and re-raising as Kitchen::ActionFailed would give a readable message instead.

Prior art: #32 already did this.