exit VFIL_searchpath upon dlopen error

Together with the previous commit, this fixes potentially misleading
error messages.

Because we called vcc_path_dlopen on all vmod_path elements
irrespective of the kind of dlopen() error, the actual root cause
could be hidden.

Example:

dlerror: .../lib/libvmod_blob/.libs/libvmod_debug.so: cannot open shared object file

when the actual error was

dlerror: .../lib/libvmod_debug/.libs/libvmod_debug.so: undefined symbol:...

lib/libvcc/vcc_vmod.c

@@ -48,7 +48,7 @@ vcc_path_dlopen(void *priv, const char *fn)
 
 	hdl = dlopen(fn, RTLD_NOW | RTLD_LOCAL);
 	if (hdl == NULL)
-		return (1);
+		return (-1);
 	pp = priv;
 	*pp = hdl;
 	return (0);