Better error message when Pinboard API is down

[Cito]

Show an error message and a retry button when the Pinboard API is down instead of simply leaving everything deactivated (see #10).

[RichardDavies]

Is there an easy way to check whether the Pinboard API is currently down?

[Cito]

Just try loading https://api.pinboard.in/v1/posts/recent in the browser.

[Cito]

Note to self: There is actually a retry button, which has been fixed in v0.2.9. Need to investigate whether something still needs to be done here.

Also: Don't forget to update DEVELOP.md when this is fixed.

[obilodeau]

Pinboard is down again for me. It would be nice to have some error message instead of everything freezing.

[Cito]

Cannot promise that I can work on this any time soon. Whoever wants to contribute, feel free to send a PR.