Improve error messages when claiming a managed gateway

[PavelJankoski]

Summary

References #7547

Changes

• Improved the error messages when claiming a managed gateway based on gRPC error status code.

Testing

Steps

1. Login to console.
2. Go to Gateways.
3. Click on Register gateway.
4. Follow the steps in the last comment of the issue.

Results

NotFound:

FailedPrecondition:

PermissionDenied:

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.