Usefully report endpoint outages

**Is your feature request related to a problem? Please describe.**
Phanpy does not currently display anything when it's unable to fetch data from the user's home instance. At present, Phanpy only reports it's unable to fetch posts when the user reloads Phanpy and initial data fetches fail.

**Describe the solution you'd like**
A modal reporting the outage would be nice. Or, well. Any message at all, really.

**Additional context**
I only found out my home instance was down by going directly to my home instance's website.