[4.4] Fixes to form validation process

[wilsonge]

Fixes hardening measure introduced in #23716

Summary of Changes

In the linked PR I forbid submitting by adding an exception into the validation process. Unfortunately I didn't make the method return false in order to make sure that message was used. This fixes that.

Alongside it improves the documentation around the getError's method. It hasn't returned any strings since 3.0 (and even maybe further - I wasn't sure about how the JException objects were exactly being passed around).

Testing Instructions

Code review

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[SniperSister]

Looks good from my side!

[richard67]

I have tested this item ✅ successfully on c246730

Code review.

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[richard67]

Looks good from my side!

@SniperSister Does this count as successful test? If so, could you mark the test result in the issue tracker, or shall I do that for you, so I can set RTC?

[SniperSister]

I have tested this item ✅ successfully on c246730

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[richard67]

RTC

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