SMT2 back-end: report error when invoking solver fails

Our previous attempt at reporting errors was dead code for `run` never
returns a negative value. When the executable isn't found the return
code was 1, else failures will depend on the solver's return code. Also,
propagate the messages produced by `run` on stderr to the error-logging
output.

Fixes: diffblue#8362

src/solvers/smt2/smt2_dec.cpp

@@ -138,10 +138,12 @@ decision_proceduret::resultt smt2_dect::dec_solve(const exprt &assumption)
   int res =
     run(argv[0], argv, stdin_filename, temp_file_stdout(), temp_file_stderr());
 
-  if(res<0)
+  if(res != 0)
   {
     messaget log{message_handler};
-    log.error() << "error running SMT2 solver" << messaget::eom;
+    std::ifstream stderr_stream(temp_file_stderr());
+    log.error() << "error running SMT2 solver: " << stderr_stream.rdbuf()
+                << messaget::eom;
     return decision_proceduret::resultt::D_ERROR;
   }