Remove ModuleNotFoundError exception check

Turns out ModuleNotFoundError was only added in Python 3.6, and since
our AppImages are built with 3.4 this causes problems.

But ModuleNotFoundError is a subclass of ImportError anyway, so just
checking for ImportError covers all cases.

src/classes/app.py

@@ -74,7 +74,7 @@ def __init__(self, *args, mode=None):
 
             # Re-route stdout and stderr to logger
             reroute_output()
-        except (ImportError, ModuleNotFoundError) as ex:
+        except ImportError as ex:
             tb = traceback.format_exc()
             QMessageBox.warning(None, "Import Error",
                                 "Module: %(name)s\n\n%(tb)s" % {"name": ex.name, "tb": tb})