MWException: Don't send headers multiple times

Ideally we wouldn't have sent headers yet, but in some weird
code paths where we output stuff raw (like Export) we might
have already sent them.

All depends on when the failure happened.

Should silence all of the "headers already sent" being seen
from exports.

Change-Id: I12e0532e93d30b2255f73a9d0e017c73e30c3e28
Merge "MWException: Don't send headers multiple times"