Drop failed sub-streams during both Get and Return.

The old code ensured that failed sub-streams would not be re-used, but
had two flaws:
1) It only checked for failed sub-streams during Return.
2) It didn't actually remove the failed sub-streams from our state.

The new code fixes these two flaws, and adds an extra test that
explains why (1) is insufficient.

PiperOrigin-RevId: 207333296