Fix exhaustiveness of error code switch in http status conversion

[merlinnot]

Description

This is only a type safety change, does not change actual behaviour. Whereas the comment before the default stated that it should never get there unless TypeScript does its job, it didn't hold true - an error was thrown in the default case, which is a branch of execution that is perfectly correct. An unreachable code is represented by a never type in TypeScript.

This PR adds a function that accepts a parameter of type never, which will not be allowed by the compiler. If the impossible happens, a readable error will be thrown.

Code sample

// This will compile
function f(x: 'a' | 'b'): void {
  switch (x) {
    case 'a':
    case 'b':
      console.log('reachable');

      return;
    default:
      assertNever(x);
  }
}

// While this will throw an error during compilation
function f(x: 'a' | 'b'): void {
  switch (x) {
    case 'a':
      console.log('reachable');

      return;
    default:
      assertNever(x);
  }
}

[thechenky]

Thanks, this looks great!