RFR: Allow transition from any old generation state to idle

[William Kemper]

A cancelled GC can return the old generation to `IDLE` from any state. Also, improve assert message.

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Commit messages:
 - Fix format specifier
 - Remove invalid assertion, improve other assertion messages

Changes: https://git.openjdk.org/shenandoah/pull/240/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=240&range=00
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/shenandoah/pull/240.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/240/head:pull/240

PR: https://git.openjdk.org/shenandoah/pull/240