Default track not in error message when branch requested

[Adam Collard]

$ snap version
snap 2.43.1+git1636.8291fc2~ubuntu16.04.1
snapd 2.43.1+git1636.8291fc2~ubuntu16.04.1
series 16
ubuntu 20.04
kernel 5.4.0-9-generic

With a default-track set in the store (confirmed with surl), I can successfully "snap install --candidate maas" and get 2.7/candidate as expected.

If I want to request a branch of the default track, I specify "snap install maas --channel=candidate/foo" expecting to get 2.7/candidate/foo

$ snap install maas --channel=candidate/foo
error: requested a non-existing branch on latest/candidate for snap "maas": foo

Note it reports for 'latest/candidate', not '2.7/candidate' as I expect (note no branch existed, just a test)

[Claudio Matsuoka (cmatsuoka)]

John Lenton notes that: "there's a store-side component to that because the error from the store doesn't let [snapd] tell the user the right thing. The quick fix would be to make snapd not tell the user the channel it was looking for, the better fix would be to make the channel mapper default-track-aware and pass in the default track from the error response (that's the bit that's missing)".