Error handling

[BartBucknill]

In multiple places (for example in verifyLogin.js) there seems to be a challenge regarding how to handle custom generated errors that we want to serve up to the client, and how to handle non-custom errors that we want to handle by serving only a generic message to the client.

Multiple .catches can cause problems as subsequent .thens get called with wrong or undefined return values (as in verifyLogin.js - bug), and later .catches may attempt to render to the client a second time, causing multiple header write errors.

In createevent.js we have addressed this problem by differentiating between custom errors and default errors based on a property present on those default errors and not on the custom errors. A more robust and readable way might be to create a custom error class, see MDN.