refs #7400. Change what is thrown.

Code/Mantid/Framework/DataObjects/src/Peak.cpp

@@ -821,7 +821,7 @@ namespace DataObjects
     auto det = getDetector();
     if(det == NULL)
     {
-      throw Mantid::Kernel::Exception::NotFoundError("Detector cannot be found.", "");
+      throw Mantid::Kernel::Exception::NullPointerException("Peak", "Detector");
     }
     return getDetector()->getPos();
   }

Code/Mantid/Framework/DataObjects/test/PeakTest.h

@@ -349,7 +349,7 @@ public:
     Peak p(inst, detectorId, wavelength);
     TSM_ASSERT_THROWS_NOTHING("Nothing wrong here, detector is valid", p.getDetectorPosition());
     p.setQLabFrame(V3D(1,1,1), 1); // This sets the detector pointer to null and detector id to -1;
-    TSM_ASSERT_THROWS("Detector is not valid", p.getDetectorPosition(), Mantid::Kernel::Exception::NotFoundError&);
+    TSM_ASSERT_THROWS("Detector is not valid", p.getDetectorPosition(), Mantid::Kernel::Exception::NullPointerException&);
   }
 
 private: