modprobe: Update error message when path is missing

Currently, modprobe fails with no output by default if the
search paths it tries are missing:

$ modprobe -S notakernel dm-crypt
$
$ modprobe -S notakernel lkjjweiojo
$

This is fairly cryptic and not at all obvious there is a problem
unless the error code is checked or verbose flags are used.
Update the error message to indicate a problem and print out the
directory that failed.
diff --git a/tools/modprobe.c b/tools/modprobe.c
index 3ba8f52..43605cc 100644
--- a/tools/modprobe.c
+++ b/tools/modprobe.c
@@ -489,11 +489,10 @@
 						const char *options) = NULL;
 
 	err = kmod_module_new_from_lookup(ctx, alias, &list);
-	if (err < 0)
-		return err;
 
-	if (list == NULL) {
-		LOG("Module %s not found.\n", alias);
+	if (list == NULL || err < 0) {
+		LOG("Module %s not found in directory %s\n", alias,
+			ctx ? kmod_get_dirname(ctx) : "(missing)");
 		return -ENOENT;
 	}