Add proper error handling if Path::Class is missing

[Photon89]

Path::Class is an optional dependency, which is only required for the Imgur.pm upload plugin. So far Shutter didn't handle it properly and you got non-fatal errors on launch if the dependency was not met:

INFO: new upload-plugin information detected - /home/photon/Downloads/shutter-master/share/shutter/resources/system/upload_plugins/upload/Imgur
Can't locate Path/Class.pm in @INC (you may need to install the Path::Class module) (@INC entries checked: /home/photon/Downloads/shutter-master/share/shutter/resources/modules /usr/lib/perl5/5.42/site_perl /usr/share/perl5/site_perl /usr/lib/perl5/5.42/vendor_perl /usr/share/perl5/vendor_perl /usr/lib/perl5/5.42/core_perl /usr/share/perl5/core_perl) at /home/photon/Downloads/shutter-master/share/shutter/resources/system/upload_plugins/upload/Imgur.pm line 77.
BEGIN failed--compilation aborted at /home/photon/Downloads/shutter-master/share/shutter/resources/system/upload_plugins/upload/Imgur.pm line 77.

ERROR: upload-plugin exists but does not work properly - /home/photon/Downloads/shutter-master/share/shutter/resources/system/upload_plugins/upload/Imgur

Also, the update-plugin dialog popped up for some milliseconds at each launch.

With this PR the dialog won't appear each launch and you get a kind of clean warning similar to other warnings for unmet optional dependencies:

WARNING: Path::Class is missing --> uploading to Imgur will be disabled!

 at ./shutter_new line 9410.
        Shutter::App::fct_init_depend() called at ./shutter_new line 201

I couldn't figure out how to get rid of the last two lines but this happens to all other warnings as well, so not related to this PR.

[Photon89]

For some reason the CI fails at the "install deps" step, it won't complete after 30 mins. Not sure if this is a temporary or a permanent issue and whether we can do something about it.