500 errors not giving error-notes (related to PR #3455)

[Cliff Skolnick <cl...@steam.com>]

A couple people have bugged me that since 1.3.3 mod_proxy does not return
the real error that occurred.  I understand that http_protocol.c was changed
to hide error-notes on account of the file system structure of servers was
being exposed via error messages, which is good.  Unfortunately this change
hurts mod_proxy by hiding important failure information.  It is a pretty
easy bug to fix specifically for mod_proxy, but while working on the fix I
noticed a comment in http_protocol.c that seems to imply a solution to
expose error-notes in certain environments would be nice.

At first I changed mod_proxy to add another note "error-expose" set to "*",
and http_protocol.c was changed to give the full error-notes when
"error-expose" was set if there were any notes.  I then thought it might be
nice to create a directive "verbose errors" that would have the same syntax
as "allow from" on a per resource basis.  Any matching address/domain would
always get the full error-notes, just like before.

Does anyone have a better idea?  If not I'll post a patch tomorrow.