[crac] RFR: Improve C/R exception printout

[Radim Vansa]

Some users might get confused by the inner exceptions reported during C/R as *suppressed* exceptions. This PR changes the printout to make it look as if the exception had multiple causes. For example the DryRunTest will report this:

jdk.crac.CheckpointException: Failed with 2 inner exceptions
Cause 1/2: java.lang.RuntimeException: should not pass
	at DryRunTest$CRResource.beforeCheckpoint(DryRunTest.java:47)
	at java.base/jdk.crac.impl.AbstractContext.invokeBeforeCheckpoint(AbstractContext.java:44)
	... (redacted)
Cause 2/2: jdk.crac.impl.CheckpointOpenFileException: /tmp/jtreg-DryRunTest6956725915963168340.tmp
	at java.base/jdk.internal.crac.JDKFileResource.lambda$beforeCheckpoint$1(JDKFileResource.java:89)
	at java.base/jdk.crac.Core.checkpointRestore1(Core.java:174)
        ... (redacted)

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Commit messages:
 - Improve C/R exception printout

Changes: https://git.openjdk.org/crac/pull/145/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=145&range=00
  Stats: 103 lines in 5 files changed: 101 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/crac/pull/145.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/145/head:pull/145

PR: https://git.openjdk.org/crac/pull/145