Fixed download file from URL error messages

- L10N now converted to string to make them work with json_encode
- Added specific error message when server doesn't allow fopen on URLs
- Fixed client side to correctly show error message in a notification
- Added OCP\JSON::encode() method to encode JSON with support for the
  OC_L10N_String values
Merge pull request #6295 from owncloud/files-fromlinkerrormessagefixes

Fixed download file from URL error messages