When composer.json contains invalid json, the resulting error is
```
Invalid value for name property in composer.json. Please refer to the Extension Validation Policy for the valid format.
```
whichs comes from https://github.com/phpbb/customisation-db/blob/4e2c5d4f2fb448d3ea10348b00c7a59352369bb9/contribution/extension/type.php#L210
Instead of having 1 large if, it should be split and different messages should be presented to the user.