Add hook point on fstab mount failure

Check return code from `mount -na`, if there where any errors give
plugins a chance to do something about it.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugin.h

@@ -43,6 +43,7 @@
 typedef enum {
 	/* Bootstrap hooks */
 	HOOK_ROOTFS_UP = 0,
+	HOOK_MOUNT_ERROR,
 	HOOK_BASEFS_UP,
 	HOOK_NETWORK_UP,
 	HOOK_SVC_UP,