Properly handle requests with malformed URIs

[leo]

Previously, requests like /%E0%A4%A caused a 500 instead of a 400:

{ URIError: URI malformed
    at decodeURIComponent (<anonymous>)
    at module.exports (/usr/src/app/node_modules/serve-handler/src/index.js:493:23)
    at trackStatusCodes (/usr/src/app/src/index.js:143:40)
    at /usr/src/app/node_modules/@zeit/metrics/http-status.js:20:10
    at module.exports.apiErrors (/usr/src/app/src/index.js:248:5)
    at <anonymous>
  [stack]: 'URIError: URI malformed\n    at decodeURIComponent (<anonymous>)\n    at module.exports (/usr/src/app/node_modules/serve-handler/src/index.js:493:23)\n    at trackStatusCodes (/usr/src/app/src/index.js:143:40)\n    at /usr/src/app/node_modules/@zeit/metrics/http-status.js:20:10\n    at module.exports.apiErrors (/usr/src/app/src/index.js:248:5)\n    at <anonymous>',
  [message]: 'URI malformed' }

Now they cause a proper 400.

[TooTallNate]

LGTM