Merge pull request #4080 from peakon/fk_error_message

Include the error message in the exception to ease debug

lib/dialects/postgres/query.js

@@ -363,6 +363,7 @@ Query.prototype.formatError = function (err) {
       table = table ? table[1] : undefined;
 
       return new sequelizeErrors.ForeignKeyConstraintError({
+        message: errMessage,
         fields: null,
         index: index,
         table: table,