[crac] RFR: Don't crash the VM if checkpointing fails

[Volker Simonis]

Currently, if checkpointing is failing (because the `criu` executable can't be find or if `criu dump` returns an error) the JVM will crash with:

$ java -XX:+CRPrintResourcesOnCheckpoint -XX:CCheckpointTo=/tmp/crac HelloWait
HelloWorld
JVM: FD fd=0 type=character: details1="/dev/pts/9" OK: inherited from process env
JVM: FD fd=1 type=character: details1="/dev/pts/9" OK: inherited from process env
JVM: FD fd=2 type=character: details1="/dev/pts/9" OK: inherited from process env
JVM: FD fd=3 type=regular: details1="/output/crac-dbg/images/jdk/lib/modules" OK: inherited from process env
CR: Checkpoint ...

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Commit messages:
 - Don't crash the VM if checkpointing fails

Changes: https://git.openjdk.org/crac/pull/25/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=25&range=00
  Stats: 26 lines in 3 files changed: 0 ins; 25 del; 1 mod
  Patch: https://git.openjdk.org/crac/pull/25.diff
  Fetch: git fetch https://git.openjdk.org/crac pull/25/head:pull/25

PR: https://git.openjdk.org/crac/pull/25