Lower log level of standard error messages from PT's.

This patch lowers the log level of error messages from PT processes from
warning to debug.

See: https://bugs.torproject.org/33005

changes/bug33005

@@ -0,0 +1,3 @@
+  o Minor bugfixes (pluggable transports):
+    - Lower the log level of standard error messages from a PT from warning to
+      info. Fixes bug 33005; bugfix on 0.4.0.1-alpha.

src/feature/client/transports.c

@@ -1858,7 +1858,9 @@ managed_proxy_stderr_callback(process_t *process,
   if (BUG(mp == NULL))
     return;
 
-  log_warn(LD_PT, "Managed proxy at '%s' reported: %s", mp->argv[0], line);
+  log_info(LD_PT,
+           "Managed proxy at '%s' reported via standard error: %s",
+           mp->argv[0], line);
 }
 
 /** Callback function that is called when our PT process terminates. The