octave.cxx: fix exception raising for newer Octave versions

- Since (at least) Octave 5.1.0, the Octave error() function now raises a C++
  exception, which if uncaught immediately exits a SWIG wrapper function,
  bypassing any cleanup code that may appear after a "fail:" label.
- This patch adds a "try { ... } catch(...) { }" block around the contents of
  SWIG wrapper functions to first execute the cleanup code before rethrowing any
  exception raised.
- It is backward compatible with earlier versions of Octave where error() does
  not raise an exception, which will still branch to the "fail:" block to
  execute cleanup code if an error is encountered.

Source/Modules/octave.cxx

@@ -567,6 +567,10 @@ public:
     Wrapper *f = NewWrapper();
     Octave_begin_function(n, f->def, iname, overname, !overloaded);
 
+    // Start default try block to execute
+    // cleanup code if exception is thrown
+    Printf(f->code, "try {\n");
+
     emit_parameter_variables(l, f);
     emit_attach_parmmaps(l, f);
     Setattr(n, "wrap:parms", l);
@@ -754,9 +758,20 @@ public:
     }
 
     Printf(f->code, "return _out;\n");
-    Printf(f->code, "fail:\n");	// we should free locals etc if this happens
+
+    // Execute cleanup code if branched to fail: label
+    Printf(f->code, "fail:\n");
     Printv(f->code, cleanup, NIL);
     Printf(f->code, "return octave_value_list();\n");
+
+    // Execute cleanup code if exception was thrown
+    Printf(f->code, "}\n");
+    Printf(f->code, "catch(...) {\n");
+    Printv(f->code, cleanup, NIL);
+    Printf(f->code, "throw;\n");
+    Printf(f->code, "}\n");
+
+    // End wrapper function
     Printf(f->code, "}\n");
 
     /* Substitute the cleanup code */