'snap run --shell snap.hello-world.env' tries to read /snap/snap/current

[Jamie Strandboge]

If I invoke snap run --shell accidentally with 'snap.' I get a confusing error message:

$ snap run --shell snap.hello-world.env
error: cannot find current revision for snap snap: readlink /snap/snap/current: no such file or directory

snappy correctly bailed but I think the message could be improved with a little input validation. Perhaps check if the specified command meets the regex for $SNAP_NAME.$COMMAND and then if it doesn't, output:

$ snap run --shell snap.hello-world.env
error: could not find 'snap.hello-world.env'. Please specify 'snap run --shell <snap name>.<command>'

or something similar.

[Zygmunt Krynicki (zyga)]

This is still happening as Jamie described. I think it is a papercut, marking the bug as such.

[Ian Johnson (anonymouse67)]

I think the priority of this should be raised somewhat, because this also happens when a snap is disabled and you try to run it:

```
$ snap disable mari0
mari0 disabled
$ snap run mari0
error: cannot find current revision for snap mari0: readlink /snap/mari0/current: no such file or directory
```

[Michael Vogt (mvo)]

I think there are two parts to this:

1. better error if the command does not look like a valid snap app
2. better error if the snap is disabled or not installed