Add explanatory error message for puls exec command

[visortelle]

Reported by @jak78

Looks like puls exec does not accept an instance name:
$ puls exec multinode pulsar-admin
stat /Users/.../.config/puls/instances/default/docker-compose.yml: no such file or directory

...
replace the error with a clear, explanatory message
...
That could be sth like:
 default instance not found. Please create one first using puls create. Alternatively, you can specify an instance with the --instance flag

[visortelle]

In the case above, multinode is treated as a command and pulsar-admin as the command's argument.

The exec command allows to specify an instance by using the --instance argument. Another option is to use puls set-default-instance <instance_name> command.

Usage: puls exec [OPTIONS]

Options:
      --instance <INSTANCE>  Pulsar instance name
      --cluster <CLUSTER>    Cluster index, e.g. 0, 1, 2
  -h, --help                 Print help
  -V, --version              Print version

I think the initial confusion comes from the exec command doesn't print help if no arguments were provided. It should be fixed.