Prevent errors from causing a crash loop

[leo]

The coverage decreased in #56, this PR brings it back to 100%.

But most importantly, if an error occurs while reading an error template, we don't try to read the error template again, but instead just render the default text error.

[mischnic]

yarn test logs some errors since this got merged (test: error for request with malformed URI). I guess that was not intended?

Error: Any error occured while checking the file
    at Object.stat (.../serve-handler/test/integration.js:595:11)
    at Object.sendError (.../serve-handler/src/index.js:29:2179)
    at module.exports (.../serve-handler/src/index.js:45:17)
Error: Any error occured while checking the file
    at Object.createReadStream (.../serve-handler/test/integration.js:1241:11)
    at Object.sendError (.../serve-handler/src/index.js:29:2534)