if ( editApi && editApi.captcha && (
		editApi.captcha.url ||
		editApi.captcha.type === 'simple' ||
		editApi.captcha.type === 'math' ||
		editApi.captcha.type === 'recaptchanocaptcha' ||
		editApi.captcha.type === 'question' 
	) ) {
		this.saveErrorCaptcha( editApi );
		return;
	}

Modified by adding "recaptchanocaptcha" No more unknown error!
Like you said now we can work on

ve.init.mw.ArticleTarget.prototype.saveErrorCaptcha

In T203478#4559177, @Esanders wrote:

Thanks, the place to start would be ve.init.mw.ArticleTarget.prototype.saveErrorCaptcha in VE.

In T203478#4559141, @Esanders wrote:

With our limited resources we have to prioritise support for Wikimedia sites over third parties, but we welcome community patches!

In T203478#4557588, @Esanders wrote:

I imagine it would violate our ToS with regards to giving away user data to a 3rd party.