(PE-2408) Add namespaces to slingshot exceptions

This makes them more easily identifiable when the code is used
in other projects.
(PE-2408) Make cli exception types more specific

Changed from `:error` and `:help` to `:cli-error` and `:cli-help`.
Also added a utility function, `without-ns`, that can be used
if the user does not care to consider the namespace of a keyword
when an exception is thrown.
Merge pull request #25 from cprice404/maint/master/PE-2408-namespace-slingshot-exceptions

(PE-2408) Add namespaces to slingshot exceptions