Improve error messages when logging into self-hosted sites

[twstokes]

Fixes #17119

Related PRs:

• Improve XML-RPC error messaging WordPressKit-iOS#464
• Improve error description WordPressAuthenticator-iOS#632

To test:

On a self-hosted site install the attached plugin (XML-RPC Tweak) which will aid in testing: xml-rpc-tweak.zip

Some steps will require the tester to log in using an invalid username and or password. This is only to overcome a limitation with how the plugin simulates certain failures.

Test 1:

1. Set XML-RPC Tweak to Mode 1
2. Log in to the self-hosted site
3. Observe error "XML-RPC services are disabled on this site."

Before	After

Test 2:

1. Set XML-RPC Tweak to Mode 2
2. Log in to the self-hosted site with an invalid username / password
3. Observe error "Unable to read the WordPress site at that URL. Tap 'Need Help?' to view the FAQ."

Before	After

Test 3:

1. Set XML-RPC Tweak to Mode 3
2. Log in to the self-hosted site with an invalid username / password
3. Observe error "There was a problem communicating with the site. An HTTP error code 401 was returned."

Before	After

Test 4:

1. Set XML-RPC Tweak to Mode 4
2. Log in to the self-hosted site
3. Observe error "Unable to read the WordPress site at that URL. Tap 'Need Help?' to view the FAQ."

Before	After (no change)

Regression Notes

1. Potential unintended areas of impact

• Regression in error messages when logging into self-hosted sites

2. What I did to test those areas of impact (or what existing automated tests I relied on)

• Executed the testing steps above.

3. What automated tests I added (or what prevented me from doing so)

• None.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[guarani]

Tested and working great! I also tested with the plugin set to "Off" mode and login to a self-hosted site worked as normal.

Code changes look good too!