Previously, we'd overwrite the errors in a circular buffer. Now that
error return traces are intended to follow a stack discipline, we no
longer have to support the index rolling over. By treating the trace
like a saturating stack, any pop/restore code still behaves correctly
past-the-end of the trace.
As a bonus, this adds a small blurb to let the user know when the trace
saturated and x number of frames were dropped.
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