DocumentMigrator exception not handled by logger

[tylersmalley]

When debugging a 7.9.0 issue with @joshdover, we discovered that the migration error was not present in the logs as would be expected. This makes understanding failed migrations on Cloud difficult if not impossible.

Example failure:

Error: Document "5a03eb37-2240-4bdd-a999-91e1d427f5bf" has property "alert" which belongs to a more recent version of Kibana [7.9.0]. The last known version is [undefined] [object Object]
    at find (/home/tyler/elastic/kibana/src/core/server/saved_objects/migrations/core/document_migrator.ts:351:13)
    at Array.find (<anonymous>)
    at nextUnmigratedProp (/home/tyler/elastic/kibana/src/core/server/saved_objects/migrations/core/document_migrator.ts:337:21)
    at applyMigrations (/home/tyler/elastic/kibana/src/core/server/saved_objects/migrations/core/document_migrator.ts:262:18)
    at DocumentMigrator.transformDoc (/home/tyler/elastic/kibana/src/core/server/saved_objects/migrations/core/document_migrator.ts:244:9)
    at transform (/home/tyler/elastic/kibana/src/core/server/saved_objects/migrations/core/document_migrator.ts:155:17)
    at Immediate.setImmediate (/home/tyler/elastic/kibana/src/core/server/saved_objects/migrations/core/migrate_raw_docs.ts:84:17)
    at runCallback (timers.js:705:18)
    at tryOnImmediate (timers.js:676:5)
    at processImmediate (timers.js:658:5)

(don't read into this particular error, it was caused by a migration added in 7.9.0 and removed before its release)

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)