Handle updated exception signature from super validate method.

[rtibbles]

Summary

• A recent change to the handling of authentication errors in the PeerImportSingleSyncValidator changed the kind of error thrown by the validate function: Better handling of validation errors when remote facility does not require passwords #11704
• This was not accounted for in the MergeUserValidator which inherits from and extends this validate method
• Updates the method to handle the ValidationError instead

References

Fixes #11768

Reviewer guidance

Follow the steps outlined in the issue - attempt to merge a user into a facility, when that user does not yet exist in that facility.

---

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

[jredrejo]

Thanks, it makes total sense

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b11.dev0_git.9.g86cdf34d.pex
Windows Installer (EXE)	kolibri-0.16.0b11.dev0+git.9.g86cdf34d-unsigned.exe
Debian Package	kolibri_0.16.0b11.dev0+git.9.g86cdf34d-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b11.dev0+git.9.g86cdf34d-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b11.dev0+git.9.g86cdf34d-0.1.0-debug.apk
TAR file	kolibri-0.16.0b11.dev0+git.9.g86cdf34d.tar.gz
WHL file	kolibri-0.16.0b11.dev0+git.9.g86cdf34d-py2.py3-none-any.whl