consistently format unreachable_package

* Unreachable package always returns as {error, {unreachable_package, Package} }
  from ``depsolver:solve``
* correct vim modelines
* add test coverage for incorrect unreachable_package response
Merge pull request #14 from opscode/mp/unreachable-package-error-format

consistently format unreachable_package so that solve always returns {error, {unreachable_package, Package} }