make mapping saturation error messages more specific

[JanReifenrath]

Currently, if all the models fail, for example because of very unusual mapping activity throughout the years, the label shown simply says "The creation of the indicator was unsuccessful.".
To give some more info to the user who might be wondering what went wrong, it would be great to add an extra edge case check that gives a different label such as "model saturation curve creation was unsuccessful".

For reference, this is what it currently sometimes looks like in the dashboard and the logging info for it.

The osm history stats in this case:

[Hagellach37]

In this particular case, it might be even better to still plot the figure and timeseries but add a textual description that no model could be fit.