Improve error messages when typeToCode throws an exception (#1450)

`typeToCode` can throw an expection, which reduces the error message to:

```
UnimplementedError: (InvalidTypeImpl) InvalidType
package:json_serializable/src/utils.dart 221:3              typeToCode
package:json_serializable/src/helper_core.dart 79:46        createInvalidGenerationError
...
```

This change improves the error message to something like this:

```
Could not generate `fromJson` code for `photos` because of type is unimplemented/unsupported/undefined.
package:picthrive_kiosk/ptclient/generated/consolidated_public.swagger.dart:2633:21
     ╷
2633 │   final List<Items> photos;
     │                     ^^^^^^
     ╵

```

Author: fromkeith

json_serializable/lib/src/helper_core.dart

@@ -77,7 +77,12 @@ $converterOrKeyInstructions
 * Set `JsonSerializable.genericArgumentFactories` to `true`
   https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonSerializable/genericArgumentFactories.html''';
   } else if (field.type != error.type) {
-    message = '$message because of type `${typeToCode(error.type)}`';
+    try {
+      message = '$message because of type `${typeToCode(error.type)}`';
+      // ignore: avoid_catching_errors
+    } on UnimplementedError catch (ex) {
+      message = '$message because type is unimplemented ($ex)';
+    }
   } else {
     final element = error.type.element?.name;
     todo =

json_serializable/test/json_serializable_test.dart

@@ -153,6 +153,7 @@ const _expectedAnnotatedTests = {
   'UnsupportedEnum',
   'UnsupportedListField',
   'UnsupportedMapField',
+  'UnsupportedNestedFunctionType',
   'UnsupportedSetField',
   'UnsupportedUriField',
   'ValidToFromFuncClassStatic',

json_serializable/test/src/_json_serializable_test_input.dart

@@ -626,3 +626,10 @@ class DateTimeUtcTestClass {
 
   DateTimeUtcTestClass(this.date);
 }
+
+@ShouldThrow('''
+Could not generate `fromJson` code for `field` because type is unimplemented (UnimplementedError: (FunctionTypeImpl) void Function()).''')
+@JsonSerializable(createToJson: false)
+class UnsupportedNestedFunctionType {
+  late List<void Function()> field;
+}