Support validation within in-line edit context

[ScopeyNZ]

Description

Currently the in-line editing editor and forms provide no detail on errors that have been added with validate() or via a form validator returned from getCMSCompositeValidator().

Investigation

See #688. Specifically #688 (comment) .

There's also an investigation branch:

https://github.com/creative-commoners/silverstripe-elemental/tree/pulls/4.1/validation-refactor

A/Cs:

• Errors added using addError($message) appear on a block level within the in-line edit form after saving an individual block
• Errors added using addError($message) appear on a block level within the in-line edit form after saving the page as a whole
• Errors added using addFieldError($field, $message) appear on a form field (holder) level within the in-line edit form after saving an individual block
• Errors added using addFieldError($field, $message) appear on a form field (holder) level within the in-line edit form after saving the page as a whole
• When saving pages, all blocks are processed and those that are valid are correctly saved.
• When saving individual blocks, the block being saved is processed and if validation messages are displayed as appropriate
• Blocks that aren't saved keep their unsaved changes
• The unsaved changes indicator remains on invalid & unsaved blocks

[GuySartorelli]

Also relevant: #329

[GuySartorelli]

When saving pages, all blocks are processed and those that are valid are correctly saved.

I don't think this is an appropriate behaviour. When saving a page, if one field is valid and another is invalid, the valid field doesn't get saved to the record. Similarly, when saving a page, an elemental block should be treated as a series of fields belonging to that page.
Otherwise you might get a scenario where one block references content from another block, and only one of those blocks is valid - we probably don't want the page to show partially-correct content.

[maxime-rainville]

Duplicate of #329

[GuySartorelli]

@maxime-rainville Wasn't this the card we're using to track fixing up the UI?
#329 is just about getting the functionality working, but the UI will not be in a 100% ideal state after that card has been finished.

[maxime-rainville]

I would rather use this card to track the UI changes related to Elemental validation

#1145