Change JSBSim MatrixException base type

Make this exception inherit std::runtime_exception, so that it’s
caught by the ‘normal’ exception cases in FG boostrap.cxx

This is the only exception I can find in the codebase, which does not
inherit from either std::exception or std::string, so this is to fix
‘unknown exception’ errors reported in the wild.

Sentry-Id: FLIGHTGEAR-1C

src/FDM/JSBSim/math/FGMatrix33.cpp

@@ -437,9 +437,7 @@ FGMatrix33 FGMatrix33::operator/(const double scalar) const
     Quot.data[5] = data[5] * tmp;
     Quot.data[8] = data[8] * tmp;
   } else {
-    MatrixException mE;
+      throw MatrixException{"Attempt to divide by zero in method FGMatrix33::operator/(const double scalar)"};
-    mE.Message = "Attempt to divide by zero in method FGMatrix33::operator/(const double scalar)";
-    throw mE;
   }
   return Quot;
 }
@@ -460,9 +458,7 @@ FGMatrix33& FGMatrix33::operator/=(const double scalar)
     data[5] *= tmp;
     data[8] *= tmp;
   } else {
-    MatrixException mE;
+      throw MatrixException{"Attempt to divide by zero in method FGMatrix33::operator/=(const double scalar)"};
-    mE.Message = "Attempt to divide by zero in method FGMatrix33::operator/=(const double scalar)";
-    throw mE;
   }
   return *this;
 }

src/FDM/JSBSim/math/FGMatrix33.h

@@ -42,6 +42,7 @@ INCLUDES
 
 #include <string>
 #include <iosfwd>
+#include <stdexcept>
 
 #include "FGColumnVector3.h"
 
@@ -64,10 +65,12 @@ CLASS DOCUMENTATION
 DECLARATION: MatrixException
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
 
-class MatrixException //: public FGJSBBase
+// changed by James to inherit std::runtime_error, so that if this
+// gets thrown, we can actually catch it.
+class MatrixException : public std::runtime_error
 {
 public:
-  std::string Message;
+    MatrixException(const std::string& msg) : std::runtime_error{msg} { }
 };
 
 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%