[Security Solution][SIEM migrations] Add timeouts and surface execution errors

[semd]

Summary

Implements controls to have more visibility over the errors, especially in the initialization phase (populate ELSER indices)

Changes

• Added timeout to the initialization phase (20 minutes).
• Added concurrency control for initialization tasks, only the first concurrent migration will trigger it, and the rest will await it.
• Added proper error handling for the ES bulk index operations of integrations and prebuilt rules ELSER indices.
• Added timeout for individual agent invocations (3 minutes)
• Added migrationsLastError server state to store the errors (not ideal, this should be moved to the migration index when we implement it) for now it's fine.
• Added the last_error in the /stats API response.
• The UI displays the last_error if it's defined.

Screenshots

Onboarding error:

Rules page error:

[elasticmachine]

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