Add error handling for GPGME imports and add logic to shell out if the module is not present

[dephekt]

This PR adds error handling for the recently added GPGME module imports. If the correct GPGME module is not available on a system, the program will fall back to the legacy method of shelling out to the GPG executable.

I removed python-gpg from the stdeb.cfg file in order to prevent unresolvable dependencies, since the target Ubuntu suite is Trusty (14.04 LTS) and python-gpg doesn't exist in Ubuntu repositories until Zesty (17.04).

Fixes #267 regression from PR #266

[micahflee]

So I'm just now looking at merging this. Do you know what the difference are between the ubuntu packages python-gpg and python-gpgme?

Only the newest ubuntu (zesty) includes python-gpg, and also only the newest debian (stretch) includes python-gpg. However older ubuntus and debians include python-gpgme. Are they the same library? If so, would it make sense to just rely on python-gpgme for now, to avoid all the extra logic?

[dephekt]

See #267 for the info on python-gpg and python-gpgme in Ubuntu (they are entirely different and not compatible). If you want to target any Ubuntu platforms prior to Zesty 17.04 it'll need that shell out logic still.

Ubuntu 14.04 LTS relies on gpgme 1.6 C libraries and they were from before python bindings were rolled into the gpgme project (in libgpgme 1.7+), so using the 1.7+ python bindings will not work against the 1.6 libgpgme as the python bindings weren't present there prior to libgpgme 1.7+

You can clone the launcher source on an Ubuntu LTS VM, install python-gpgme and try to run the app and see that it dies in a fire for more reasons than just the module name being different.

[micahflee]

Sorry it took so long to get around to reviewing this, but looks good!