Avoid null values for CheckedFromJsonException.message

...by using `toString()` with unrecognized error types.

Fixes #745

Author: kevmoo

_test_yaml/test/yaml_test.dart

@@ -68,7 +68,7 @@ builders:
 - a
 - b
 ''': r'''
-line 2, column 1 of file.yaml: Unsupported value for "builders".
+line 2, column 1 of file.yaml: Unsupported value for "builders". type 'YamlList' is not a subtype of type 'Map<dynamic, dynamic>' in type cast
   ╷
 2 │ ┌ - a
 3 │ └ - b
@@ -88,7 +88,7 @@ builders:
   a:
     target: 42
   ''': r'''
-line 3, column 13 of file.yaml: Unsupported value for "target".
+line 3, column 13 of file.yaml: Unsupported value for "target". type 'int' is not a subtype of type 'String' in type cast
   ╷
 3 │     target: 42
   │             ^^

json_annotation/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## 3.1.1-dev
 
+- Avoid `null` values for `CheckedFromJsonException.message` by using 
+  `toString()` with unrecognized error types.
 - Added a helpful `UnrecognizedKeysException.toString()`.
 - Fixed doc comments for `JsonSerializable.genericArgumentFactories`.
 

json_annotation/lib/src/checked_helpers.dart

@@ -117,7 +117,7 @@ class CheckedFromJsonException implements Exception {
       }
       return message;
     }
-    return null;
+    return error.toString();
   }
 
   @override