Improve message for `snap install purchasable-snap`

snapd
1

This is about improving the error message when trying to install a snap that needs to be purchased first.

The current error message when running:

$ snap install kiosc

reads like this:

error: cannot perform the following tasks:
- Download snap "kiosc" (16) from channel "stable" (please buy kiosc before installing it.)

which is mostly fine except that it does not tell the user how to purchase it.

We already add suggestions when a login is required by appending: ... (see 'snap help login') so we could do something similar here:

error: cannot perform the following tasks:
- Download snap "kiosc" (16) from channel "stable" (please buy kiosc before installing it, see 'snap help buy')

WDYT?

(edit: trivial branch here https://github.com/snapcore/snapd/compare/master...mvo5:snap-buy-error-improvement?expand=1)

2

in theory we have a flag in info.MustBuy to error even before starting the install, but it requires having retrieved the list of purchases so I don’t remember when it’s available

3

I doubled checked, MustBuy is set only by Store.SnapInfo and Store.Find but we could optionally set it SnapAction too (controlled by the options).

4

Related: Bug #1763071 “Error message installing a paid snap as unauthenticated user”.