add more specific error message when app or stage config not found

With the current behavior, invoking pulsar with incorrect parameters leads to the following non-specific ArgumentError:

```
$ pulsar my_app my_stage my_cap_task
/Users/matt/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/pulsar-0.3.1/lib/pulsar/helpers/clamp.rb:178:in `validate': ArgumentError (ArgumentError)
```
or
```
$ pulsar my_stage my_cap_task
/Users/matt/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/pulsar-0.3.1/lib/pulsar/helpers/clamp.rb:178:in `validate': ArgumentError (ArgumentError)
```
With the PR, pulsar will complain in a more specific way which might help the user in constructing the proper pulsar invocation, particularly when they have the wrong # of parameters for the context, eg:
```
$ pulsar my_stage my_cap_task
/Users/matt/dev/pulsar/lib/pulsar/helpers/clamp.rb:178:in `validate': no pulsar config available for app=my_stage, stage=my_cap_task (ArgumentError)
```