Escape contents of source document snippet for JsonLocation._appendSourceDesc()

[cowtowncoder]

Currently a leading snippet of contents parsed is included in JsonLocation description; all characters are just included as-is. But it would make sense to ensure that non-printable characters (control characters in particular) are escaped and not just printed out, as these descriptions are often simply printed out as-is into application logs and control characters (like linefeeds, tabs) may be problematic in such cases.