t5551: fix expected error output

We should probably get rid of the check of message instead, but in
the meantime this should do.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'sp/smart-http-content-type-check' into pu

* sp/smart-http-content-type-check:
  t5551: fix expected error output