[crac] RFR: Print better diagnostics for spawning CRIU [v2]

[Jan Kratochvil]

> It was always a mystery why it does not work this time as there are no error messages printed.
> Still may system call errors are currently not reported but those errors at least do not happen commonly.
> In fact `os::exec_child_process_and_wait` should be more verbose but that is already a part of OpenJDK so I did not modify it.
> It would be much easier if `criuengine.c` was in C++. I was considering to switch it but then I wrote it already in C.
> I understand this code will become obsolete after planned integration of CRIU into JVM but it may not yet be soon enough.

Jan Kratochvil has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - print error messages in C++
 - Merge branch 'crac' into crac-error
 - Print better diagnostics for spawning CRIU.

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Changes: https://git.openjdk.org/crac/pull/97/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=97&range=01
  Stats: 68 lines in 2 files changed: 62 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/crac/pull/97.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/97/head:pull/97

PR: https://git.openjdk.org/crac/pull/97