Python: use the module run().__doc__ as a pretty description.

src/libcalamares/PythonJob.cpp

@@ -248,8 +248,11 @@ PythonJob::prettyName() const
 QString
 PythonJob::prettyStatusMessage() const
 {
+    if ( m_description.isEmpty() )
         return tr( "Running %1 operation." )
                 .arg( QDir( m_workingPath ).dirName() );
+    else
+        return m_description;
 }