verify: extract "manifest" constant into variable The "manifest" label that's used in error messages will instead be the directory path for subdirectory manifests (not the root manifest), so let's extract the constant to a variable already to make future patches simpler.

verify: use similar language for missing manifest and file revisions When a changeset refers to a manifest revision that's not found in the manifest log, we say "changeset refers to missing revision X", but when a manifest refers to file revision that's not found in the filelog, we say "X in manifests not found". The language used for missing manifest revisions seems clearer, so let's use that for missing filelog revisions too.