...which helps us differentiate between a drive that's not mounted vs.
not a dir better in log messages. We were already doing that a bit in
diskfile.py, and it seems like a useful distinction; let's do it more.
While we're at it, remove some log translations.
Related-Change: I941ffbc568ebfa5964d49964dc20c382a5e2ec2a
Related-Change: I3362a6ebff423016bb367b4b6b322bb41ae08764
Change-Id: Ife0d34f9482adb4524d1ab1fe6c335c6b287c2fd
Partial-Bug: 1674543