Fixed fs error during theme install deleting active theme (#12688)

closes https://github.com/TryGhost/Ghost/issues/12506

- adds an error handler that will rename the backup folder to the original name if the newly uploaded theme wasn't saved successfully

core/frontend/services/themes/storage.js

@@ -48,6 +48,7 @@ module.exports = {
         }
 
         let checkedTheme;
+        let renamedExisting = false;
 
         return validate.checkSafe(zip, true)
             .then((_checkedTheme) => {
@@ -58,6 +59,7 @@ module.exports = {
             .then((themeExists) => {
                 // CASE: move the existing theme to a backup folder
                 if (themeExists) {
+                    renamedExisting = true;
                     return getStorage().rename(shortName, backupName);
                 }
             })
@@ -86,6 +88,20 @@ module.exports = {
                     theme: toJSON(shortName, checkedTheme)
                 };
             })
+            .catch((error) => {
+                // restore backup if we renamed an existing theme but saving failed
+                if (renamedExisting) {
+                    return getStorage().exists(shortName).then((themeExists) => {
+                        if (!themeExists) {
+                            return getStorage().rename(backupName, shortName).then(() => {
+                                throw error;
+                            });
+                        }
+                    });
+                }
+
+                throw error;
+            })
             .finally(() => {
                 // @TODO: we should probably do this as part of saving the theme
                 // CASE: remove extracted dir from gscan happens in background