This commit replaces the use of "sanity" with more inclusive
naming. When "sanity" is used in a more general sense either
"validity" or "quick" is used.
For this include a seed-error in the /v2/debug?aspect=seeding debug API as well.
* daemon: support seed-error in /v2/debug?aspect=seeding
* cmd/snap: print seed-error if there is one in snap debug seeding
* cmd/snap: adjust seed-error test to new -- placeholder
This, in conjunction with the associated change to `snap debug seeding` client
will help in determining when to output "-" for a time / duration that can't be
calculated because the start time is 0 and the end time is an actual time for
example, where now we output unexpected things like seeding taking 292 years,
instead we could just output `seed-completion: -`
Signed-off-by: Ian Johnson <ian.johnson@canonical.com>
