fix: Return structured errors from GFAL2 storage operations

[chrisburr]

New PR for #8439 which was merged too early.

Summary

• Return error dictionaries with errno codes from exists() and getFileMetadata() in GFAL2_StorageBase
• Captures gfal2.GError separately to extract the e.code (errno)
• Allows consumers to distinguish between different error types (e.g., ENOENT vs EACCES)

See: https://gitlab.cern.ch/lhcb-dirac/LHCbDIRAC/-/merge_requests/1870

Motivation

When a storage element returns HTTP 401 (Permission Denied / Authentication Error), dirac-dms-check-fc2se incorrectly reports files as "missing at SEs". This is because all files in metadata["Failed"] are treated as "not existing" regardless of the actual error type.

By returning structured error information with errno codes, downstream code can properly handle permission denied errors separately from file-not-found errors, avoiding false positives.

Changes

The error format changes from:

failed[url] = repr(e)  # string

to:

failed[url] = {"error": repr(e), "errno": e.code}  # dict with errno

Backward Compatibility

Downstream code should handle both the old string format and new dict format during the transition period:

if isinstance(err_info, dict):
    err_code = err_info.get("errno")
else:
    err_code = None  # Legacy string format

BEGINRELEASENOTES

*DataManagement
CHANGE: Expose type of error from GFAL2_StorageBase.getFileMetadata

ENDRELEASENOTES