Model validation failures now reported in UI.

"New" application helper method #explain_errors takes a model instance
and renders its associated error messages as HTML. What *is* new about
it is that it includes correct CSS styling for Bootstrap 3, unlike
previous versions of the helper. (Specs for this?)

Feature spec "Member cannot create invalid post" re-enabled and updated
to include correct error message text.

RSpec: 97 examples, 0 failures, 1 pending
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 31 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5