Shows more useful error message if no plugins are installed
If no plugins are installed, zathura will show a message like:
$ ./zathura
error: could not open plugin directory: /usr/local/lib64/zathura
I propose to show a more useful error message, like:
$ ./zathura
error: plugin directory not found: /usr/local/lib64/zathura
Did you forgot to install any plugin?