Why this change?
The current shape of errors returns the error messages after it has been
translated but there are cases where we want to customize the error
messages and the current way return only translated error messages is
making customization of error messages difficult. If we
wish to have the error messages in complete sentences like
"`some_property` property must be present in #link 1", this is not
possible at the moment with the current shape of the errors we return.
What does this change do?
This change introduces the `ThemeSettingsObjectValidator::ThemeSettingsObjectErrors`
and `ThemeSettingsObjectValidator::ThemeSettingsObjectError` classes to
hold the relevant error key and i18n translation options.
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