[crac] RFR: Print better diagnostics for spawning CRIU

[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.

-------------

Commit messages:
 - Print better diagnostics for spawning CRIU.

Changes: https://git.openjdk.org/crac/pull/97/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=97&range=00
  Stats: 139 lines in 2 files changed: 100 ins; 7 del; 32 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