EHN: add INTEGRITY_VIOLATION as message code

proto/caosdb/entity/v1/main.proto

@@ -321,6 +321,10 @@ enum MessageCode {
   // out of range. The integer range is an implementation detail and thus this
   // can happen even though the API allows the value.
   MESSAGE_CODE_INTEGER_VALUE_OUT_OF_RANGE = 58;
+
+  // This entity caused an unexpected integrity violation. This is a strong
+  // indicator for a server bug. Please report.
+  MESSAGE_CODE_INTEGRITY_VIOLOATION = 59;
 }
 
 // Messages are used by server's or client's to transport *transient*