Print the error when an exception is thrown due to a non-zero return …

…code from calling `trac-admin`.

trac/tests/functional/testenv.py

@@ -195,8 +195,9 @@ def _tracadmin(self, *args):
         if proc.returncode:
             print(out)
             logfile.write(out)
-            raise Exception('Failed with exitcode %s running trac-admin ' \
-                            'with %r' % (proc.returncode, args))
+            raise Exception("Failed while running trac-admin with arguments %r.\n"
+                            "Exitcode: %s \n%s"
+                            % (args, proc.returncode, out))
 
     def start(self):
         """Starts the webserver, and waits for it to come up."""