The .write function was having issues with large files, the
connection would close or sometimes there would be errors.
I changed thefunction to act more like smb_relay and it works better.

Due to the vagaries of various Metasploit module description viewers, we
can't guarantee things like lists and bullet points render right.
Descriptions should avoid using these things.

Instead of a cryptic exception, let the user know if the HTTP target
isn't actually asking for WWW-Authenticate.

There are likely many more opportunities to catch errors, but this is
the most obvious.