Add a new function mrb_exc_protect().

`mrb_exc_protect()` takes two C functions, `body` to be executed first,
and `resc` to be executed when an error happens during `body` execution.
Since `mrb_exc_protect()` should be compiled with the proper compiler,
we will not see the problem like #5088 that was caused by `setjmp()` and
`throw` mixture.

src/error.c

@@ -555,6 +555,29 @@ mrb_argnum_error(mrb_state *mrb, mrb_int argc, int min, int max)
 #undef FMT
 }
 
+MRB_API mrb_value
+mrb_exc_protect(mrb_state *mrb, mrb_value (*body)(mrb_state*, void*), void *a,
+                mrb_value (*resc)(mrb_state*, void*, mrb_value), void *b)
+{
+  struct mrb_jmpbuf *prev_jmp = mrb->jmp;
+  struct mrb_jmpbuf c_jmp;
+  mrb_value v = mrb_undef_value();
+
+  MRB_TRY(&c_jmp) {
+    mrb->jmp = &c_jmp;
+    v = body(mrb, a);
+  } MRB_CATCH(&c_jmp) {
+    if (mrb->exc) {
+      v = resc(mrb, b, mrb_obj_value(mrb->exc));
+      mrb->exc = NULL;
+    }
+  } MRB_END_EXC(&c_jmp);
+
+  mrb->jmp = prev_jmp;
+
+  return v;
+}
+
 void mrb_core_init_printabort(void);
 
 int