[BugFix] Include underlying cause in HdfsFsManager copy error messages (backport #73414)

[mergify]

Why I'm doing:

HdfsFsManager.copyFromLocal / copyToLocal wrap the underlying Hadoop exception in a StarRocksException but only put the source / destination paths into the new message, dropping the cause's message.

Callers that surface only StarRocksException#getMessage to the user can lose all useful diagnostic information. The motivating case is automated cluster snapshot upload to remote object storage:

ClusterSnapshotCheckpointScheduler.java
    errMsg = "upload image failed, err msg: " + e.getMessage();

When the underlying S3/OSS upload fails (AccessDenied, NoSuchBucket, SocketTimeoutException, SignatureDoesNotMatch, FileAlreadyExistsException, etc.), the user only sees the following in information_schema.cluster_snapshot_jobs.error_message:

upload image failed, err msg: Failed to copy local /opt/starrocks/fe/meta/image to s3://tenant-bucket/.../meta/image/automated_cluster_snapshot_xxx

…with no hint of the real reason. Operators currently have to grep fe.log for the Exception while copy local … stack trace to find what actually went wrong.

What I'm doing:

• Append e.getMessage() to the StarRocksException wrapper message in both copyFromLocal and copyToLocal (for both InterruptedIOException and the generic Exception paths), so the real cause is visible at the call site without losing the existing wrapper for log greppability.
• Drive-by: fix the missing space before to local in copyToLocal's error string (previously produced Failed to copy /pathto local /dest).
• Add unit tests testCopyToLocalIncludesCauseMessage and testCopyFromLocalIncludesCauseMessage covering both methods.

The underlying cause is already attached to the StarRocksException and still gets logged with full stack trace via LOG.error — this change only adds the cause's message to the user-visible message string.

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
  • This pr needs auto generate documentation
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.1
  • 4.0
  • 3.5
  • 3.4

    ---

    This is an automatic backport of pull request [BugFix] Include underlying cause in HdfsFsManager copy error messages #73414 done by Mergify.