Handle missing dependencies of plugins during plugin_load_try

Instead of letting m64py crash with an OSError when the loading of a plugin
fails, it should better create an error message but continue working with the
remaining plugins.

An error can for example look like this:

Frontend: ERROR: failed to load plugin /usr/lib/x86_64-linux-gnu/mupen64plus/mupen64plus-video-glide64mk2.so: libpng15.so.15: cannot open shared object file: No such file or directory
Merge pull request #13 from fayvel/failure_plugin

Handle missing dependencies of plugins during plugin_load_try