use a base iz3_exception class for exceptions raised during interpolation

Using a base exception class, derived from z3_exception, makes it possible to
recover gracefully if something goes wrong during the computation of
interpolants.

src/interp/iz3exception.h

@@ -0,0 +1,28 @@
+/*++
+Copyright (c) 2015 Microsoft Corporation
+
+Module Name:
+
+    iz3exception.h
+
+Abstract:
+
+    Base class for exceptions raised by interpolation routines 
+
+Author:
+
+Notes:
+
+--*/
+#ifndef _IZ3EXCEPTION_H_
+#define _IZ3EXCEPTION_H_
+
+#include "z3_exception.h"
+#include "error_codes.h"
+
+class iz3_exception: public default_exception {
+public:
+    iz3_exception(const std::string &msg): default_exception(msg) {}
+};
+
+#endif