tasks: make error a json

also, handle case where we never got to handle task exit cleanly

migrations/20190830204531-tasks-rename-errorMessage-errorJson.js

@@ -0,0 +1,15 @@
+'use strict';
+
+exports.up = function(db, callback) {
+    db.runSql('ALTER TABLE tasks CHANGE errorMessage errorJson TEXT', [], function (error) {
+        if (error) console.error(error);
+        callback(error);
+    });
+};
+
+exports.down = function(db, callback) {
+    db.runSql('ALTER TABLE tasks CHANGE errorJson errorMessage TEXT', [], function (error) {
+        if (error) console.error(error);
+        callback(error);
+    });
+};