Adds "Error Message" to Fields UI

- Adds the Error Message field to the Fields screens, to allow viewing and
  editing that property on each field.
- Moves "description" of a field to the top field to edit.
- Adds "description" field to the fields "view" screen.

Additionally, this PR makes the verbiage of the flash messages consistent,
using the terms "field" and "fields" instead of "input" and "input filters".