[D8][UX] Add inline form errors

This is part of #378 as it is now a feature included in D8 core (d.org issue):

This was something that tested really well on the D8 UX study, and we should consider adding to Backdrop as well.

User interface changes

• Form validation error messages are displayed below the respective form fields that have triggered them.
• A brief indication of the number of errors, and a link to each error is displayed in the messages area at the top of the page.

Contrib modules

• Drupal 7 inline messages module

  

• Drupal 6 (and 7) ife module

  

• Inline Form Errors unofficial and very experimental Backdrop contrib module by @daggerhart:

  

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~~ Advocate for this issue: @klonos ~~