Log error message when slot fails validation

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 882cb84..30ac131 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -590,6 +590,7 @@
              * continue booting from slot 0.
              */
         }
+        BOOT_LOG_ERR("Image in slot %d is not valid!", slot);
         return -1;
     }