blaze_util_posix: handle killpg failures

In case Bazel JVM server is stuck, it would be useful to know whether
the SIGKILL was sent successfully or not.

If not, log out the error message from errno.

src/main/cpp/blaze_util_posix.cc

@@ -726,7 +726,12 @@ void ReleaseLock(BlazeLock* blaze_lock) {
 
 bool KillServerProcess(int pid, const blaze_util::Path& output_base) {
   // Kill the process and make sure it's dead before proceeding.
-  killpg(pid, SIGKILL);
+  errno = 0;
+  if (killpg(pid, SIGKILL) == -1) {
+    BAZEL_DIE(blaze_exit_code::LOCAL_ENVIRONMENTAL_ERROR)
+        << "Attempted to kill stale server process (pid=" << pid
+        << ") using SIGKILL: " << GetLastErrorString();
+  }
   if (!AwaitServerProcessTermination(pid, output_base,
                                      kPostKillGracePeriodSeconds)) {
     BAZEL_DIE(blaze_exit_code::LOCAL_ENVIRONMENTAL_ERROR)