Improve error message for provider not installed.
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Issue body
### What needs to get done
ATM running `pack` before having done `lxd init` gives:
```
$ charmcraft pack
Provider is required but not installed. Do you wish to install provider and configure it with the defaults? [y/N]:
```
It would be more helpful if the error message stated more clearly what needs doing and what entering `y` will in fact do.
How about:
```
$ charmcraft pack
lxd is present but not initialized. Did you forget to run `lxd init`? Do you wish to initialize lxd with its default profile? [y/N]:
```
### Why it needs to get done
error message as it stands is not informative
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