make it easier to figure out if there is an error.

[jvanasco]

I am trying to handle a situation where i can call multiple 3rd party command line functions and need to tell if they worked or not (the apps may not be installed).

The current version is silent on errors. An status_code is set on the result object, however there is no documentation on what this status code means or what expected values could be.

In my case, the error code was 127 and is being set manually by package. Thankfully the source was brief, and I quickly saw this https://github.com/kennethreitz/envoy/blob/master/envoy/core.py#L216-L218

It would be great if a future version simply noted what good/bad values are for status codes -- or at least documented what known errors would be. I dug through several stdlib packages and could not find a corresponding 127 error.