Warn the user when a nondeterministic task is detected. (#339)

* WARN instead of FATAL for object hash mismatches, push error to driver

* Document the callback signature for object_table_add/remove

* Error table

* Wait for all errors in python test

* Fix doc

* Fix state test

src/common/state/error_table.cc

@@ -0,0 +1,23 @@
+#include "error_table.h"
+#include "redis.h"
+
+void push_error(DBHandle *db_handle,
+                DBClientID driver_id,
+                int error_index,
+                size_t data_length,
+                unsigned char *data) {
+  CHECK(error_index >= 0 && error_index < MAX_ERROR_INDEX);
+  /* Allocate a struct to hold the error information. */
+  ErrorInfo *info = (ErrorInfo *) malloc(sizeof(ErrorInfo) + data_length);
+  info->driver_id = driver_id;
+  info->error_index = error_index;
+  info->data_length = data_length;
+  memcpy(info->data, data, data_length);
+  /* Generate a random key to identify this error message. */
+  CHECK(sizeof(info->error_key) >= UNIQUE_ID_SIZE);
+  UniqueID error_key = globally_unique_id();
+  memcpy(info->error_key, error_key.id, sizeof(info->error_key));
+
+  init_table_callback(db_handle, NIL_ID, __func__, info, NULL, NULL,
+                      redis_push_error, NULL);
+}