Add an exception in DecayTreeFit when constraint cannot be applied

Closes #408 (closed) . Needs DaVinci!759 (merged)

Phys/DecayTreeFitter/src/lib/DecayChain.cpp

@@ -10,7 +10,9 @@
 \*****************************************************************************/
 #include "DecayChain.h"
 #include "FitParams.h"
+#include "GaudiKernel/GaudiException.h"
 #include "InteractionPoint.h"
+#include "Kernel/ParticleID.h"
 #include "ParticleBase.h"
 #include <algorithm>
 
@@ -175,6 +177,11 @@ namespace DecayTreeFitter {
     if ( changed ) {
       m_dim = 0;
       m_constraintlist.clear();
+    } else {
+      std::cout << "ERROR: Constraint of particle " << pid.pid()
+                << " was not applied as it is missing in the decay tree. Check your options." << std::endl;
+      throw GaudiException( "To-be constrained particle is not in chain.", "DecayTreeFitter::DecayChain",
+                            StatusCode( 856 ) );
     }
   }