Standard USAGE format
Writes usage & general errors to stderr
**Handles N targets**
URL validation (url.Parse doesn't do validation)
Generally more comments throughout
Returns 504 Gateway Timeout when all targets fail (rather than 404 Not
Found)

1. When 'failed' is closed !ok will be true -- we weren't properly
   hitting that case and the connection would stay open.
2. Cancels the request when a bad response code is hit.