fix: convert segment index error to warning with graceful skip

[igoroctaviano]

When a SEG frame is missing SegmentIdentificationSequence in both per-frame and shared functional groups, the code now logs a warning and skips that frame instead of throwing an error that aborts the entire segmentation loading.

Error fixed:

Error: Could not retrieve the segment index. Aborting segmentation loading.

Change:

- throw new Error('Could not retrieve the segment index. Aborting segmentation loading.');
+ console.warn('Could not retrieve the segment index for frame ' + i + ', skipping this frame.');
+ continue;

This follows the same pattern already used for missing imageIds (lines 587-589) and makes SEG loading more resilient to non-standard or incomplete DICOM SEG files.

DICOM Compliance:

Per DICOM PS3.3 C.8.20.3.1, SegmentIdentificationSequence is required for LABELMAP segmentations, but graceful degradation is preferred over hard failure when encountering non-compliant files.

Related:

• Upstream PR: fix(adapters): add null checks for PerFrameFunctionalGroupsSequence in SEG loading cornerstonejs/cornerstone3D#2963
• Upstream Issue: [Bug] checkOrientation crashes when PerFrameFunctionalGroupsSequence is undefined cornerstonejs/cornerstone3D#2962