Sorry to reopen this, if some error pages are not overridable we need to provide an API for the embedder to know that, we cannot expect apps to know which pages are overridable and which are not, especially if the API is still present but doesn't do anything / fails when used.
Bug 1696841 Comment 15 Edit History
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Sorry to reopen this, if some error pages are not overridable we need to provide an API for the embedder to know when an error can be overridden, we cannot expect apps to know which pages are overridable and which are not, especially if the API is still present but doesn't do anything / fails when used.