tests: commitlog_test: test_allocation_failure: improve error reporting
tests: commitlog_test: test_allocation_failure: improve error reporting

We're seeing the following error from test from time to time:
  fatal error: in "test_allocation_failure": std::runtime_error: Did not get expected exception from writing too large record

This is not reproducible and the error string does not contain
enough information to figure out what happened exactly, therefore
this patch adds an exception if the call succeeded unexpectedly
and also prints the unexpected exception if one was caught.

Refs #4714

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20191215052434.129641-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla