Fix JSON.stringify "bug" with strings containing \u2028 or \u2029
See http://www.thespanner.co.uk/2011/07/25/the-json-specification-is-now-wrong/
Use Zotero.Utilities.varDump instead of JSON.stringify for logging
This reduces discrepancy between the two logs and will yield better results when zotero/zotero#599 gets accepted (i.e. you can log Error objects)