`omdb db inventory collections show` fails unhelpfully when collection is missing

[davepacheco]

root@oxz_switch0:~# omdb db inventory collections show 7dc6f0da-020a-43e5-a7ff-2ea02f14354f
note: database URL not specified.  Will search DNS.
note: (override with --db-url or OMDB_DB_URL)
note: using DNS server for subnet fd00:1122:3344::/48
note: (if this is not right, use --dns-server to specify an alternate DNS server)
note: using database URL postgresql://root@[fd00:1122:3344:109::3]:32221,[fd00:1122:3344:105::3]:32221,[fd00:1122:3344:10b::3]:32221,[fd00:1122:3344:107::3]:32221,[fd00:1122:3344:108::3]:32221/omicron?sslmode=disable
note: database schema version matches expected (150.0.0)
Error: fetching collection 7dc6f0da-020a-43e5-a7ff-2ea02f14354f

Caused by:
    Internal Error: failed runtime check: "collections.len() == 1"

It should report a more explicit message.