binman: Add a consistent way to report errors with fit

Add a new function to handling reporting errors within a particular
subnode of the FIT description. This can be used to make the format of
these errors consistent.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
diff --git a/tools/binman/ftest.py b/tools/binman/ftest.py
index 3d672e6..c09bf36 100644
--- a/tools/binman/ftest.py
+++ b/tools/binman/ftest.py
@@ -5303,7 +5303,7 @@
         """Check handling of an FDT map when the section cannot be found"""
         with self.assertRaises(ValueError) as exc:
             self._DoReadFileDtb('224_fit_bad_oper.dts')
-        self.assertIn("Node '/binman/fit': Unknown operation 'unknown'",
+        self.assertIn("Node '/binman/fit': subnode 'images/@fdt-SEQ': Unknown operation 'unknown'",
                       str(exc.exception))
 
     def test_uses_expand_size(self):