verify: include "manifest" prefix in a few more places
We include the "manifest" prefix on most other errors, so it seems
consistent to add them to the remaining messages too. Also, having the
"manifest" prefix will be more consistent with having the directory
prefix there when we add support for treemanifests. With the
"manifest" at the beginning, let's remove the now-redundant
"manifest" in the message itself.
[hgk]
path = /usr/share/mercurial/hgk