Add differing exit codes for differing end states

[gaenserich]

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When scripting, it is often useful to get different exit codes for different sorts of failures (e.g. 0 for everything went correct, 1 for unrecognized options, 2 for an authentication error, 3 for lack of pay wall credentials, etc.). Curl, for instance, provides some extremely useful exit codes that help scripters to provide other contingent reactions like reattempting the download, parsing the url for errors, etc. Moreover, having these error codes would be more elegant than redirecting stderr to stdout and grep'ing the resulting string for key phrases.

[dstftw]

Duplicate of #882.