Fix syncing when password-less flow is enabled

[jredrejo]

Summary

This PR fixes several issues found when testing a scenario where users can sync and login without passwords, according to the server settings:

1. NOT_SPECIFIED is correctly sent when there's no password
2. Provides to the user more details with the error excepcion in case the syncing task fails
3. Failed sync users are removed from the list of synced users

It also improves the handling of the syncing task , avoiding possible conflicts if more than a browser is connected.

References

Closes: #8325

Reviewer guidance

Try to provision one device syncing normal users from a server that allow users to login without password.

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md