Some WMS VS plots create service exception when called for "empty" region

[Joern Ungermann]

With a flight path outside the "valid" region of a data set, the generic plots cause a Service Exception instead of a simple white plot. At least the error message should be clarified.

[Joern Ungermann]

Fixed by 6fc51e67bed9

[Joern Ungermann]

Fixed by 6fc51e67bed9