Feature: Add to_s to base Error module

In order for our HumanErrors to behave more like standard Ruby `Exception`s,
it's helpful for them to be able to give a descriptive message about what
happened.

By default, Ruby uses the `message` and `to_s` methods on the exception to
retrieve this information.

--------------------------------------------------------------------------------
Change-Id:     I0652d6c8ff75e29c54b9d8a36fc115d9431d4418
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Version Bump to 1.7.0