8339487: ProcessHandleImpl os_getChildren sysctl call - retry in case of ENOMEM and enhance exception message

Reviewed-by: alanb, lucy, rriggs

src/java.base/unix/native/libjava/ProcessHandleImpl_unix.c

@@ -528,7 +528,7 @@ jint unix_getChildren(JNIEnv *env, jlong jpid, jlongArray jarray,
      * position integer as a filename.
      */
     if ((dir = opendir("/proc")) == NULL) {
-        JNU_ThrowByNameWithLastError(env,
+        JNU_ThrowByNameWithMessageAndLastError(env,
             "java/lang/RuntimeException", "Unable to open /proc");
         return -1;
     }