Include spawned command into stderr if Popen fails

Instead of just traceback saying that e.g. "No such command" without
information which command it was tried to spawn, stderr will now include
what was tried to be run. This makes diagnostic easier.

nxdtest

@@ -174,7 +174,8 @@ def main():
             env.update(envadj)
             p = Popen(t.argv, env=env, stdin=devnull, stdout=PIPE, stderr=PIPE, bufsize=1, **kw)
         except:
-            stdout, stderr = '', traceback.format_exc()
+            stdout = ''
+            stderr = 'run %r\n%s' % (t.argv, traceback.format_exc())
             sys.stderr.write(stderr)
             status['error_count'] += 1
         else: