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.h

@@ -0,0 +1,50 @@
+#ifndef ERROR_TABLE_H
+#define ERROR_TABLE_H
+
+#include "db.h"
+#include "table.h"
+
+typedef struct {
+  DBClientID driver_id;
+  unsigned char error_key[20];
+  int error_index;
+  size_t data_length;
+  unsigned char data[0];
+} ErrorInfo;
+
+/** An error_index may be used as an index into error_types and
+ *  error_messages. */
+typedef enum {
+  /** An object was added with a different hash from the existing
+   *  one. */
+  OBJECT_HASH_MISMATCH_ERROR_INDEX = 0,
+  /** The total number of error types. */
+  MAX_ERROR_INDEX
+} error_index;
+
+/** Information about the error to be displayed to the user. */
+static const char *error_types[] = {"object_hash_mismatch"};
+static const char *error_messages[] = {
+    "A nondeterministic task was reexecuted."};
+
+/**
+ * Push an error to the given Python driver.
+ *
+ * @param db_handle Database handle.
+ * @param driver_id The ID of the Python driver to push the error
+ *        to.
+ * @param error_index The error information at this index in
+ *        error_types and error_messages will be included in the
+ *        error pushed to the driver.
+ * @param data_length The length of the custom data to be included
+ *        in the error.
+ * @param data The custom data to be included in the error.
+ * @return Void.
+ */
+void push_error(DBHandle *db_handle,
+                DBClientID driver_id,
+                int error_index,
+                size_t data_length,
+                unsigned char *data);
+
+#endif