annotator: don't call handleError for non-errors

Cancelling an editing action rejects a promise, causing handleError to
be called. This probably isn't desireable, seeing as it currently calls
console.error(...). In addition (and more immediately important)
console.log(...) and friends can't accept non-serializable inputs such
as annotation _local objects in mocha-phantomjs, causing the tests to
fail.

Instead, we only actually hook up the handleError promise rejection
handler for real errors that come from the storage layer.