over: gracefully show invalid ibx_id

over: gracefully show invalid ibx_id

While "public-inbox-extindex --gc" invocations try to ensure
proper ordering, it is still possible for users to change
the `inboxes' tables via sqlite3(1) or similar means.  So
show a "missing://ibx_id=$ibx_id" placeholder to avoid undefined
variable warnings.

URLs such as "imaps://..." will eventually be supported as
eidx_keys, so having a URL-like "missing://" as a placeholder
probably makes sense.