Please implement consistent error messages for all error conditions, especially caused by invalid input data. Please catch and handle exceptions (weight json parser).
For beeing able to use it within the GUI I would prefer an option to get the error in some machine readable format like JSON:

{
    "errorMessage": "some human readable error message",
    "location": {
        "inputName": "query",
        "lineStart": 1,
        "lineEnd": 1,
        "columnStart": 3,
        "columnEnd": 5
    }
}