Revert 'Include spawned command into stderr if Popen fails'

This reverts commit 34e96b1d. Reason for revert is that since bd1333bb
(log += title and argv for ran testcase) we always emit details of
to-be-run command to log in the beginning of testcase run.

nxdtest

@@ -184,8 +184,7 @@ def main():
             env.update(envadj)
             p = Popen(t.argv, env=env, stdin=devnull, stdout=PIPE, stderr=PIPE, bufsize=1, **kw)
         except:
-            stdout = ''
-            stderr = 'run %r\n%s' % (t.argv, traceback.format_exc())
+            stdout, stderr = '', traceback.format_exc()
             sys.stderr.write(stderr)
             status['error_count'] += 1
         else: