Move QKeychain::NoBackendAvailable error handling to KeychainChunk class

Move QKeychain::NoBackendAvailable error handling to KeychainChunk class

Originally this was in the WebFlowCredentials class. Since we've abstracted everything
from there already, let's also move this in case some other code may use
KeychainChunk::ReadJob prior to WebFlowCredentials.

Signed-off-by: Michael Schuster <michael@schuster.ms>