fence_vmware_rest: improve exception handling in send_command()

If an exception occurs, simply raise it. pycurl's perform() method can
generate a pycurl.error object, which does not support indexing and
attempting to do so will generate an exception that hides the original
exception.

Also, don't assume that the remote will return a JSON formatted response.
If it doesn't, a exception will occur accessing result which will not
raise the intended exception.