Improve error handling

[Fetz]

Currently the demo page error handling sucks. As well the application error handling is not the best.

Demo page

• add toast error message
• validate if the json is a valid or not
• validate if depth is bigger than 0 and integer
• target is a valid get path

commandline, JS

• validate if the json is a valid or not
• validate if depth is bigger than 0 and integer
• target is a valid get path