Detect when the snapshot repo bucket cannot be found and return a clear error message

[peternied]

Description

Fix for this error because the snapshot repo doesn't exist:

2024-09-05 21:56:33,737 INFO o.o.m.MetadataMigration [main] Starting Metadata Migration
2024-09-05 21:56:33,758 INFO o.o.m.c.Migrate [main] Command line arguments {0}
2024-09-05 21:56:33,758 INFO o.o.m.c.Migrate [main] Running Metadata worker
2024-09-05 21:56:35,523 INFO o.o.m.c.ClusterProviderRegistry [main] Found snapshot resource reader for version: Elasticsearch 7.10.2
2024-09-05 21:56:38,340 INFO o.o.m.c.ClusterProviderRegistry [main] Found remote writer for version: OpenSearch 2.9.0
2024-09-05 21:56:38,677 INFO o.o.m.c.Migrate [main] Using transformation com.rfs.transformers.Transformer_ES_7_10_OS_2_11@3a1f2a1
2024-09-05 21:56:38,730 INFO c.r.w.MetadataRunner [main] Migrating the Templates...
2024-09-05 21:56:39,514 ERROR o.o.m.c.Migrate [main] Unexpected failure
java.lang.StringIndexOutOfBoundsException: String index out of range: -5
        at java.base/java.lang.String.substring(String.java:1841) ~[?:?]
        at com.rfs.common.S3Uri.<init>(S3Uri.java:9) ~[RFS.jar:?]
        at com.rfs.common.S3Repo.findRepoFileUri(S3Repo.java:63) ~[RFS.jar:?]
        at com.rfs.common.S3Repo.getSnapshotRepoDataFilePath(S3Repo.java:119) ~[RFS.jar:?]
        at com.rfs.version_es_7_10.SnapshotRepoData_ES_7_10.fromRepo(SnapshotRepoData_ES_7_10.java:34) ~[RFS.jar:?]
        at com.rfs.version_es_7_10.SnapshotRepoProvider_ES_7_10.getRepoData(SnapshotRepoProvider_ES_7_10.java:20) ~[RFS.jar:?]
        at com.rfs.version_es_7_10.SnapshotRepoProvider_ES_7_10.getSnapshotId(SnapshotRepoProvider_ES_7_10.java:59) ~[RFS.jar:?]
        at com.rfs.models.GlobalMetadata$Factory.getJsonNode(GlobalMetadata.java:37) ~[RFS.jar:?]
        at com.rfs.models.GlobalMetadata$Factory.fromRepo(GlobalMetadata.java:67) ~[RFS.jar:?]
        at com.rfs.worker.MetadataRunner.migrateMetadata(MetadataRunner.java:23) ~[RFS.jar:?]
        at org.opensearch.migrations.commands.Migrate.execute(Migrate.java:59) [MetadataMigration.jar:?]
        at org.opensearch.migrations.MetadataMigration.main(MetadataMigration.java:37) [MetadataMigration.jar:?]

Issues Resolved

• https://opensearch.atlassian.net/browse/MIGRATIONS-1978

Check List

• New functionality includes testing
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.28%. Comparing base (d3c9d99) to head (e2129d9).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #929      +/-   ##
============================================
+ Coverage     71.19%   71.28%   +0.08%     
- Complexity     1939     1942       +3     
============================================
  Files           365      365              
  Lines         14058    14060       +2     
  Branches        809      809              
============================================
+ Hits          10008    10022      +14     
+ Misses         3587     3575      -12     
  Partials        463      463              

Flag	Coverage Δ
gradle-test	64.73% <100.00%> (+0.12%)	⬆️
python-test	88.55% <ø> (ø)

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[chelma]

LGTM, thanks!