RedMine 6549: deal with some problems with parsing JSON

We didn't Log() anything useful when we failed to parse a JSON file; so improve the Log()ging of that and plug a leak in each evalfunction that did it.

Our JSON parser would barf on \u{hex digits} which is valid in JSON as a denotation for a Unicode character.
It seems more prudent to warn about such unhandled uses (and do something vaguely sensible with them) rather than failing; after all, if the user isn't using the field that contained the unrecognised escape, it's no problem at all, whereas rejecting the whole file because we don't know what to do about a few bytes of it is sad.