[FIX] replace exception handling with proper check

in ff centroided the precondition of findNearest was handled by adding
a try/catch around it instead of properly checking the pre-condition
itself. this commit replaces the try/catch with a corresponding check
[FIX] fixed try/catch misuse for findNearest also in MassTraceDetection
[FIX] addendum to 8fef18dda7b904850981f303ec5964eb5826a5b2 (findNearest - try/catch - fix)
Merge pull request #911 from aiche/fix/ff-centroided-exception-miuse

[FIX] FFCentroided replace exception handling with proper check