fix(static): cache error responses for 5 minutes

Author: trieloff

src/openwhisk/static.js

@@ -44,6 +44,7 @@ function error(message, code = 500) {
     headers: {
       'Content-Type': 'text/html',
       'X-Static': 'Raw/Static',
+      'Cache-Control': 'max-age=300',
     },
     body: `${message}`,
   };