qemu-img: Fix convert, amend error messages for unknown options

Message quality regressed in commit dc523cd.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/qemu-img.c b/qemu-img.c
index 5af6f45..82d9078 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1554,8 +1554,7 @@
         if (options) {
             qemu_opts_do_parse(opts, options, NULL, &local_err);
             if (local_err) {
-                error_report("Invalid options for file format '%s'", out_fmt);
-                error_free(local_err);
+                error_report_err(local_err);
                 ret = -1;
                 goto out;
             }
@@ -3001,8 +3000,7 @@
     if (options) {
         qemu_opts_do_parse(opts, options, NULL, &err);
         if (err) {
-            error_report("Invalid options for file format '%s'", fmt);
-            error_free(err);
+            error_report_err(err);
             ret = -1;
             goto out;
         }