Better error message in JSON parser on \'

Here's an example @sgoldlust used when working on documentation:

"resp-syntax": "{
    \"result\": 0,
    \"text\": \"Info about IPv4 shared network \'floor13\' returned\"
}"

Our parser reported error Bad Escape, whithout really saying what is wrong. I've debugged the issue and the problem came up to be it doesn't allow escaping apostrophe.

This is overly strict and should be relaxed. If we somehow decide that it's not allowed (why?), then at the very least we need to improve the error to show which character is not allowed to be escaped.

After discussion with @fdupont it seems the only thing to do is to improve the error logs.