Raise a clear error when the stage is ommitted

Resolves #542 - 'When no servers are defined, a weird filtering error
message is displayed'

Attempting to run a task with just `cap deploy` will now prompt for the
command to be re-run with the stage:

`Stage not set, please call something such as `cap production deploy`,
where production is a stage you have defined.`

It's now straightforward to implement a `default_stage` variable and
load the `config/deploy/:stage` file - however this has always felt like
a risky move to me, especially with the old multi-stage implementation
defaulting to `production`, so it's something I'd rather not see
included.