improve tests for lib/api/new.coffee

[kylemac]

in the process of upgrading to the new version of sprout on #644, i've found that there are a few use-cases we could add to to improve our coverage and integration with sprout. I'll attempt to keep a list here and contribute these back.

• roots.new() with a default template set
• roots.new() with a template that doesn't exist
• roots.new() with a template that has an error with it
• roots.new() when roots-base isn't currently installed
• roots.new() when you pass overrides to your ouput