Display error when HTTP authentication isn't configured

Its a common installation failure to set auth.type = HTTP, but
then not configure the reverse proxy server or servlet container
to actually perform the user authentication.

Help the new administrator out by telling them what might be wrong,
and display an example Apache configuration block they can use to
get things working.

Change-Id: I8523bdd80be74cd7ce3b8f94c91cdd3a6cab3853
Signed-off-by: Shawn O. Pearce <sop@google.com>
2 files changed