URL event source: make URL fetcher more fault-tolerant

[todbot]

Currently the URL fetcher will report an explicit failure if a URL fetch fails. This could happen for any number of network reasons (including host PC just waking from sleep). It corrects on the next successful fetch, but still, why does it fail to begin with, how can we ameliorate it?

Some things to try/test:

1. Observe the "online"/"offline" events in Electron and only act when appropriate to do so
2. Add simple retry feature on network-issue type of failures
3. Create better status message for users
4. Switch fetcher tool from needle to got (simpler API, smaller install?, better failure modes)