DimDescribedService ctor should check format string as datalogger does

[dneise]

DimDescribedService should check the validity of the format string and refuse to start if the format string is invalid.

Otherwise it can happen that a non-godlike developer makes a typo in the format string. The dim-service still builds and starts nicely, but the datalogger, which is checking the validity of the format string, stops with an exception.

Due to the automatic restart of each of our FACT++ programs the reason for the continuous restarting of the datalogger is very hard to see in the screen. But this is just a side note.