Improve error message when attempting to refreseh a local snap

[Alex Lowe]

This was originally reported in snapcraft by ~sed-i. I'm moving this over as it's about the snap cli.

https://github.com/snapcore/snapcraft/issues/4190

### What needs to get done

I have a snap installed from a local file
```
$ which obsidian
/snap/bin/obsidian
```
Now I see it in the store
```
$ snap info obsidian
name: obsidian
summary: Obsidian
publisher: –
store-url: https://snapcraft.io/obsidian
license: unset
description: |
  Obsidian
commands:
  - obsidian
refresh-date: 2022-08-02
channels:
  latest/stable: 1.3.5 2023-06-01 (12) 113MB classic
  latest/candidate: ↑
  latest/beta: ↑
  latest/edge: ↑
installed: 0.15.9 (x1) 77MB -
```
But if I try to refresh it I get an obscure error message:
```
$ sudo snap refresh obsidian
error: local snap "obsidian" is unknown to the store, use --amend to proceed anyway
```
I happen to remember I installed it from file, but the error message could be more informative.

### Why it needs to get done

Reduce user confusion around refreshing snaps.

[Sergio Cazzolato (sergio-j-cazzolato)]

Hi, thanks for raising this.