module-jack-tunnel: Properly propagate error from dlopen()

dlopen() does not set errno on failure, rather you're supposed to call
dlerror() to get the latest error. dlerror() return a string so
instead return -ENOENT from weakjack_load_by_path().

Depending on errno weakjack_load() could think it successfully loaded
the library, and later module-jack-tunnel would crash because it call
a NULL function pointer.

src/modules/module-jack-tunnel/weakjack.h

@@ -101,7 +101,7 @@ static inline int weakjack_load_by_path(struct weakjack *jack, const char *path)
 
 	hnd = dlopen(path, RTLD_NOW);
 	if (hnd == NULL)
-		return -errno;
+		return -ENOENT;
 
 	pw_log_info("opened libjack: %s", path);