This change improves the error message when a channel split fails with the full split state and avoids a performance issue WRT listing all the split indices in the error message as the string conversion is expensive.

Instead of listing all splits, since we've already sorted them all, we simply list the count and the first and last split indices. This gives us enough information to see what happened to cause the failure, which the current message lacks. The length and first and last would let a debugger determine if the range was complete or not.

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