Unify error messaging for forms

In de0c235cd9067b48803d6dc632f117913ace8fe2 we duplicated the bit of
code in our form partials that displays the errors that prevented the
instance from being saved. To keep things DRY, I am now moving that
similar code to a partial that both forms simply render.

Closes #38.

Change-Id: I7dd117683a19e4946242d6fad3cb2c821d3d261f