CLI: 500 error when publishing a package is not handled correctly.

[scott-taubman]

If a 500 error is thrown when publishing a package, the CLI does not return a user friendly error. Instead, it dumps the HTTP response text, which is not very helpful. Errors of any type should be handled gracefully, with as useful of a message as we can display being shown to the user.

For an example of how to reproduce this, see #222