Prevent InvocationError on non-existent files

exit code for the function will be 1 if the previous command failed
if/then condition intercepts that

Change-Id: Icf43e72c65e6c63d95aef8d289b689a200d1eaeb

bin/docstheme-build-translated.sh

@@ -112,10 +112,13 @@ function add_language_index_to_original {
 
 function recover_rst_files {
     for f in `find $DIRECTORY/source -name '*.rst'`; do
-        [ -f $f.backup ] && mv $f.backup $f
+        if [ -f $f.backup ]; then
+            mv $f.backup $f
+        fi
     done
 }
 
+
 function remove_pot_files {
     # remove newly created pot files
     rm -f ${DIRECTORY}/source/locale/*.pot