extension: allow running without the backend, improve error message

[karlicoss]

At the moment if backend isn't running, user ends up with cryptic "ERROR: Failed to fetch". At the very list, could point to readme and mention they need to run the backend.

In addition, there is some value in running against just local browser history too, so we could be defensive and work with them even without the backend visits. This will require a change somewhere here

Although not sure all queries are possible with local history APIs, but looks doable

For hierarchical queries, this will also require a change to the way visits are treated, at the moment local visits are considered 'boring. Perhaps a toggle to treat certain sources as 'interesting'?

(see https://news.ycombinator.com/item?id=23672393)

[karlicoss]

Some work on it here.. #163
I guess the only remaining bit is the 'toggle' to visualize the hierarchy

[karlicoss]

Even more work here! #164

[karlicoss]

More work here #165, basically everything that works for backend visits also works for in-browser visits now