fix(afs): catch error when enabling persistence

[Maistho]

Checklist

• Docs included?: no
• Test units included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

Instead of getting an "Uncaught Promise" error in console, this will set the persistanceEnabled$ observable properly

[jamesdaniels]

Nice one, I was planning on doing that this evening. Thanks!