use global variables $std{err,out} instead of constants
standardize exception handling and exit with non-zero status
fail when source key is not specified or does not exist
=== 1.1.30 (2013-05-15)

* Changed:  Improved error handling -- all commands now exit with non-zero exit
            code on error.  [Alexis Midon / alexism@github / Pull request #11]

* Changed:  s3cp properly fails   when source key is not specified or does not exist.
            [Alexis Midon / alexism@github / Pull request #11]