When a NotSnapError is returned right now there is no context
provided what went wrong. This makes debugging issues difficult
when the user just reports:
```
2022-11-23T14:47:57Z ERROR "/var/lib/snapd/snaps/.local-install-130452292" is not a snap or snapdir
```
This commit creates a constructor `snap.NewNotSnapErrorWithContext()`
that provides some context like:
```
cannot process snap or snapdir: open non-existing-file: no such file or directory
cannot process snap or snapdir: directory "empty-dir" is empty
cannot process snap or snapdir: directory "invalid-dir" is invalid
cannot process snap or snapdir: file "not-snap" is invalid header [110 111 116 45 97 45 115 110 97 112])
```
3ec4381aa2