Add process creation error check

So that we get a helpfull error message if process creation fails,
otherwise cbmc can continue and fail or hang later in the processing.
Note that showing an error message should be the expected behaviour
in the case where the user specifies a solver which is not available.

Author: thomasspriggs

src/util/piped_process.cpp

@@ -233,6 +233,8 @@ piped_processt::piped_processt(const std::vector<std::string> &commandvec)
   // recognize that the child process has ended (but maybe we don't care).
   CloseHandle(child_std_OUT_Wr);
   CloseHandle(child_std_IN_Rd);
+  if(!success)
+    throw system_exceptiont("Process creation failed.");
 #  else
 
   if(pipe(pipe_input) == -1)