Followups mostly to r75906.

Followups mostly to r75906.

* Improved logic around error messages, and understandability of those messages.

* Removed FIXME on Content-Length header. When we serve the file
this way, it seems to be necessary to explicitly set the Content-Length. I tried
removing it and unsurprisingly it did not appear.

I am unsure what RoanKattouw is referring to when he says PHP can do this.
How can PHP know what the Content-Length is going to be, unless we set this explicitly?