Invalid field validation errors are unintuitive, hard to spot and correct, especially for editors who are not very technical or on touch screen

[toddpadwick]

Is your feature request related to a problem? Please describe.
I often get clients struggling, when they have validation errors in their document. Most of the time, they just don't know that there are validation errors and can't figure out why it won't let them publish. The issue is even more frequent when tabs are used. if a client is in another tab, and tries to publish it does not tell them which field and in which tab the error is present.

This is the issue demonstrated in two screenshots: the first when you're in the default tab, that does not contain the invalid field. the second, when you are in the right tab, but the invalid field is only marked with a very tiny icon that does not show any error on touch screen devices.

Describe the solution you'd like

1. tab should be marked with an alert icon if it contains invalid fields.
2. invalid fields error should show bottom right in red, clearly indicating to the user why they can't publish. (works on touch screen devices)
3. error per field is shown underneath the field, making it very clear there is an error and what the issue is.