Make error messages clearer when network is offline or interrupted

[e271828-]

Right now we produce error messages like:

• Can't find variable: reset
• Can't find variable: execute

If resources cannot be loaded, e.g. if user goes out of service in the midst of a session.

1. JS SDK calls should be wrapped in JS, return comprehensible error message ("could not load resources from network")
2. We should look at robustifying this:

• watchdog timer at N seconds (15?) if webview comes up but JS fails to load. (can be done in JS: setTimeout for 15s, check if window.hcaptcha exists)