adding error class for failed diagonalization

src/error.hpp

@@ -102,6 +102,21 @@ class OutOfMemoryError : public Error {
    std::string msg;
 };
 
+/**
+ * @class NoDiagonalizationError
+ * @brief Failed diagonalization of a matrix
+ */
+class NoDiagonalizationError : public Error {
+public:
+   explicit NoDiagonalizationError()
+      {}
+   virtual ~NoDiagonalizationError() {}
+   virtual std::string what() const {
+      return std::string("NoDiagonalizationError:"
+                         " Failed mass matrix diagonalization");
+   }
+};
+
 }
 
 #endif