Log a better error upon failing to open an fd in path watcher

I've been struggling with some intermittent failures, so I'm hoping the
code returned from `open` might shed some light on the issue.

native/path_watcher.mm

@@ -164,7 +164,7 @@ static NSMutableArray *gPathWatchers;
 
     int fd = open([path fileSystemRepresentation], O_EVTONLY, 0);
     if (fd < 0) {
-      NSLog(@"WARNING: Could create file descriptor for path '%@'", path);
+      NSLog(@"WARNING: Could not create file descriptor for path '%@'. Error code %d.", path, fd);
       return NO;
     }