Displays validation error messages on control panel forms

[wesleybl]

Fixes: #5274

Now the error is displayed:

To works properly, it needs: plone/plone.restapi#1771

But it can be merged separately without any problems.

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[davisagli]

Could you resolve the merge conflict?

[davisagli]

Would it make sense to move some of this to a helper function that can be used here and in other components instead of duplicating the logic?

[wesleybl]

Done.

[wesleybl]

@ichim-david I ran into the error in the "As editor, I can unlock a locked page" test again. I remember you found a solution to the problem:

#5933 (comment)

Do you intend to make this fixe?

[wesleybl]

Could you resolve the merge conflict?

@davisagli done.

[ichim-david]

@ichim-david I ran into the error in the "As editor, I can unlock a locked page" test again. I remember you found a solution to the problem:

#5933 (comment)

Do you intend to make this fixe?

@wesleybl yup I do ... time to dust up the old pull request I didn't add, April was not a good month for Volto work unlike March :)

[ichim-david]

@ichim-david I ran into the error in the "As editor, I can unlock a locked page" test again. I remember you found a solution to the problem:

#5933 (comment)

Do you intend to make this fixe?

@wesleybl only left it there for 1 month .. if you would like to test it locally the locking test here is the pull request where I made the changes necessary to avoid errors after running it 20 times in a row as I've given you the comment on how todo it
#5998

[wesleybl]

@davisagli @sneridagh can you take a look please?