Add error log when attempting to upgrade

I was attempting to test upgrades on MacOS, and was a bit confused at the ECONNREFUSED error.

Having this log would help others avoiding troubleshooting this until it is supported.

pingora-core/src/server/transfer_fd/mod.rs

@@ -179,6 +179,7 @@ pub fn get_fds_from<P>(_path: &P, _payload: &mut [u8]) -> Result<(Vec<RawFd>, us
 where
     P: ?Sized + NixPath + std::fmt::Display,
 {
+    log::error!("Upgrade is not currently supported outside of Linux platforms");
     Err(Errno::ECONNREFUSED)
 }