systemc: Improve handling of empty process handles.

Most had checks, but didn't print any message. throw_it needed a check
as well.

Change-Id: I916c837112f9b27852583f01b3e16a6f53d5e7ca
Reviewed-on: https://gem5-review.googlesource.com/c/12440
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/ext/utils/sc_report.hh

@@ -33,7 +33,7 @@
 #include <exception>
 #include <string>
 
-#include "systemc/ext/core/sc_time.hh"
+#include "../core/sc_time.hh"
 
 namespace sc_core
 {