Improve error handling

[sbrudz]

Improve the way the node server handles errors by following the best practice recommendations below:

• Handle errors centrally; not within middleware -- https://github.com/i0natan/nodebestpractices/blob/master/sections/errorhandling/centralizedhandling.md
• Catch unhandled promise rejections https://github.com/i0natan/nodebestpractices/blob/master/sections/errorhandling/catchunhandledpromiserejection.md