Fix wrong user input in geotagging offset and/or give users errormessages
When setting an offset from a picture with the geotagging plugin, darktable expects input of exactly this format: "hh:mm:ss". When the user an other format (eg. "hh:mm") darktable simply ignores it without giving the user an error message in either GUI or console.
It would be more user friendly when darkatble would fix alternative input Formats internally ("hh:mm" ==> "hh:mm:00") and give the user a GUI message, if it is not able to parse user input.