If call to fetch BreezeWiki hosts list fails, use a default list

[KevinPayravi]

IWB currently calls https://bw.getindie.wiki/instances.json every 24 hours to fetch a list of working BreezeWiki mirrors, and caches that data in storage. This is done so that BreezeWiki mirrors can be quickly added/removed, as some mirrors tend to go down or disappear.

However, if the network call to get the instances fails the first time, the user will have no mirrors whatsoever to choose from (see https://github.com/KevinPayravi/indie-wiki-buddy/issues/618 for example). We should have a default list that loads if the network call fails and no BreezeWiki instances are stored in the extension's storage.