tasks: make error a json

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

migrations/schema.sql

@@ -211,8 +211,8 @@ CREATE TABLE IF NOT EXISTS tasks(
     type VARCHAR(32) NOT NULL,
     percent INTEGER DEFAULT 0,
     message TEXT,
-    errorMessage TEXT,
-    result TEXT,
+    errorJson TEXT,
+    resultJson TEXT,
     creationTime TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
     PRIMARY KEY (id));