Remove stack trace from user errors

[victorlin]

Express' default error handler always logs a full stack trace to stderr, even for expected client-side errors like 404s. This adds significant noise (10+ lines per error) to production server logs.

Slack thread for context.

Checklist

• Tested locally with curl -I -X POST -H 'accept: text/html' http://localhost:5000/

  Before:

   [verbose]       Sending NotFoundError: Not Found error as HTML with Express' default error handler
   NotFoundError: Not Found
       at file:///…/nextstrain.org/src/routing/errors.js:25:36
       at Layer.handle [as handle_request] (/…/nextstrain.org/node_modules/express/lib/router/layer.js:95:5)
       at trim_prefix (/…/nextstrain.org/node_modules/express/lib/router/index.js:317:13)
       at /…/nextstrain.org/node_modules/express/lib/router/index.js:284:7
       at router.process_params (/…/nextstrain.org/node_modules/express/lib/router/index.js:335:12)
       at next (/…/nextstrain.org/node_modules/express/lib/router/index.js:275:10)
       at file:///…/nextstrain.org/src/routing/canary.js:21:12
       at Layer.handle [as handle_request] (/…/nextstrain.org/node_modules/express/lib/router/layer.js:95:5)
       at trim_prefix (/…/nextstrain.org/node_modules/express/lib/router/index.js:317:13)
       at /…/nextstrain.org/node_modules/express/lib/router/index.js:284:7
  

  After:

   [verbose]       Sending NotFoundError: Not Found error as plain text
   NotFoundError: Not Found
  

• Checks pass

• Check if changes affect the resource index JSON revision

[jameshadfield]

Thanks!