Enhancement: Visual improvement to errors in input prompts

We've adjusted the input prompts to show a visually less prominent text below the input field.
Also, error messages now appear with a small delay, so that those happening during
typing get ignored (e.g. trailing whitespace is not allowed in folder names and previously caused
an error to show on every typed blank).

https://github.com/owncloud/web/issues/1906
https://github.com/owncloud/web/pull/3240
