#13 Better error handling of invalid private keys

[Jason Fulghum]

If we try to use an invalid private key (ex: the private key for the EC2 certificate, not for the EC2 Key Pair) to execute a remote command or publish files to a remote host, the error message that comes back just says that the command or file copy failed without providing the user enough information to figure out what went wrong. Users can start Eclipse from the command line to get more verbose logging and see what's happening, but they shouldn't have to rely on that output from the command line.

We should make sure we're passing as much useful information back in those error messages as possible so that the user can easily figure out what they need to do to get things working.