`snap set` silently fails when given an invalid key

[Kyle Fazzari]

Config keys (the keys used by `snap set/get` and `snapctl set/get`) must follow a valid pattern. For example, underscores aren't valid. And you can see that by trying to pass it:

$ snap get my-snap invalid_key
error: invalid option name: "invalid_key"

$ snapctl get invalid_key
error: error running snapctl: invalid option name: "invalid_key"

However, `snap set` doesn't validate like these other commands. If you run `snap set my-snap invalid_key=foo` (assuming `my-snap` has a configure hook), it returns nothing, as if it succeeded. But it didn't: `invalid_key=foo` never made it into the transaction.

[Kyle Fazzari (kyrofa)]

Fix here: https://github.com/snapcore/snapd/pull/3280