mkimage: Add a few more messages for FIT failures

Add messages to make it clearer which part of the FIT creation is failing.
This can happen when an invalid 'algo' property is provided in the .its
file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/fit_image.c b/tools/fit_image.c
index 923a975..8a18b1b 100644
--- a/tools/fit_image.c
+++ b/tools/fit_image.c
@@ -36,8 +36,10 @@
 
 	tfd = mmap_fdt(params->cmdname, tmpfile, size_inc, &ptr, &sbuf, true,
 		       false);
-	if (tfd < 0)
+	if (tfd < 0) {
+		fprintf(stderr, "Cannot map FDT file '%s'\n", tmpfile);
 		return -EIO;
+	}
 
 	if (params->keydest) {
 		struct stat dest_sbuf;