Add sensible string representation of ParseError objects

[claudijo]

I noticed that our error logs are filled with thousands of "[Object object]" due to the ParseError object not having a sensible string representation. Errors ripple down to the Express.js error log handler, which calls console.error(err.stack || err.toString()). In our case, these are typically 209 Invalid session token errors, as we just migrated to using revocable sessions. The proposed fix in this pull request is to add a toSting() method for ParseErrors.

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[flovilmart]

Thanks for the PR and sorry for the late response! We'Ll get that in for 1.10!