fix(image-upload): Fix swallowing of errors for filesystem

This patch fixes the swollowing of the actual error message that appears
when a file fails to move, after being uploaded to Hedgedoc on an
instance that is using the upload-method `filesystem` active.

This became apparent when the error messages provided by some users,
where less than helpful.

As a solution the error message of the copy command was added to the
error that is output to the console.

https://community.hedgedoc.org/t/image-upload-fail-docker/439

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

lib/web/imageRouter/filesystem.js

@@ -22,7 +22,7 @@ exports.uploadImage = function (imagePath, callback) {
   try {
     fs.copyFileSync(imagePath, path.join(config.uploadsPath, fileName))
   } catch (e) {
-    callback(new Error('Error while moving file'), null)
+    callback(new Error(`Error while moving file: ${e.message}`), null)
     return
   }
   callback(null, (new URL(fileName, config.serverURL + '/uploads/')).href)