JCMA should advise user when target instance is locked

Issue Summary

When the target Jira instance in Cloud is locked, due to an incomplete migration, the error message supplied does not lead the user or Support to the true problem or the resolution. 

Steps to Reproduce

1. Create a situation that locks the target Cloud instance
   1. This is usually done by starting a migration, and then canceling it.

Expected Results

1. Preflight checks will complete, but
2. the Migration will advise the user that the process cannot be completed due to a locked instance.

Actual Results

1. Preflight checks do not complete, with an unrelated message about App checks
2. Migration cannot move forward, and situation regarding lock in the database is unknown.

The below exception is thrown in the application log file:

2024-02-01 16:28:20,957+0000 pool-92-thread-336 ERROR jiraadmin 988x10977x1 1234567 10.02.03.04,127.0.0.1 /rest/migration/latest/check/a6634d676aece0caf18fd062a4e5608468a2f1b3 [c.a.j.migration.preflight.AsyncCheckExecutor] Pre-flight execution error occurred for check type AppLicenseCheck
java.util.concurrent.ExecutionException: com.atlassian.jira.migration.httpclient.exceptions.UnexpectedStatusCodeException: Received an unexpected status code; expected [200], but received 404:  

Workaround

This error can be triggered by a need to refresh tokens (handled in MIG-1774), or by a need to unlock the instance.

To refresh your tokens, see the relevant Suggestion here: https://jira.atlassian.com/browse/MIG-1774 and the workaround article here: renew auth token for migration assistant apps

To have the database edit made to unlock the target instance, please open an Issue with Migrations Support; this edit must be made on the backend by Support.