Improve error message on invalid request start-line

[Chris-Barrett-FW]

If the HTTP message begins with whitespace, the resulting error is "invalid HTTP version". This felt misleading, or at least less helpful than it could be, since the version (HTTP/1.1) in the start-line was fine. If the message could instead either, ideally call out leading whitespace (as it may be subtle to spot), or at least instead say something like "invalid start-line", or "doesn't match HTTP format", or similar, I think it could help save people some time in tracking down this particular issue.