Fix generate-kconfig-udlr.py for Python3

Python3 does not expose an attribute `message` on `Exception`.  However,
`Exception` can implicitly convert to a string, and produces the
message.  Switch to that.

similar/main/generate-kconfig-udlr.py

@@ -343,7 +343,7 @@ class Main:
 			# show the full traceback.
 			if os.getenv('DXX_KCONFIG_UDLR_TRACEBACK') is not None:
 				raise
-			sys.stderr.write('error: %s\n' % e.message)
+			sys.stderr.write('error: {!s}\n'.format(e))
 			sys.exit(1)
 		self.write_generated_text(target,generated_text)