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
Don't installing conflicting mime type for *.*64

There is already an mime type for Nintendo64 roms called application/x-n64-rom.
This one has to be extended instead of installing a conflicting type. Otherwise
only one of the desktop entries for mupen64plus-ui-console or m64py works and
not both.