Fix formpost error message

I broke it when I made a MimeInvalid error more debuggable. Now the
more-opaque message is back for users, which is important because it
doesn't contain any user input and thus can't be part of XSS or
anything.

Change-Id: I5172ab4b5858521865d780097a360b5bf4902dab
Add etag to normal-object COPY requests

This ensures there's no silent data corruption between
objA --> proxy --> (objB1, objB2, objB3).

SLO and DLO COPY requests still don't have any etag checking, but
that's because they don't know what the MD5 of the concatenated
segments is.

Also fix sysmeta tests as part of all this.

Change-Id: Ic97e758d8c0bb832b88384169ba26a926bc944ab