[FIX] fixed copy-paste mistake in error message

(However, the error message itself also seems dubious to me: Couldn't "NaN" be used to represent a missing double value?)
Merge pull request #802 from hendrikweisser/fix-toppbase-message

[FIX] fixed copy-paste mistake in TOPPBase error message
[DOC,STYLE] documented exception, improved formating
Merge branch 'develop' into feature/AASequence_refactoring