[rest] add a more verbose error message for invalid header counts

test/functional/interface_rest.py

@@ -279,7 +279,7 @@ class RESTTest (BitcoinTestFramework):
         # Test number parsing
         for num in ['5a', '-5', '0', '2001', '99999999999999999999999999999999999']:
             assert_equal(
-                bytes(f'Header count out of acceptable range (1-2000): {num}\r\n', 'ascii'),
+                bytes(f'Header count is invalid or out of acceptable range (1-2000): {num}\r\n', 'ascii'),
                 self.test_rest_request(f"/headers/{num}/{bb_hash}", ret_type=RetType.BYTES, status=400),
             )