Improve self-hosted sign-in error logging

[crazytonyli]

Add some detailed error logs in the self-hosted sign in process.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 27.1 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	33325
	Version	PR #25814
	Bundle ID	org.wordpress.alpha
	Commit	2335142
	Installation URL	27pvate9lpm28

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	33325
	Version	PR #25814
	Bundle ID	com.jetpack.alpha
	Commit	2335142
	Installation URL	5cl2d1meq4p90

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.