Raise error if target storage class is AbstractFileSystem

[cisaacstern]

Recently observed by @jbusecke and @rsignell-usgs, if the config file is not picked up, we are not erroring out early enough.

[cisaacstern]

Per conversion with @rsignell-usgs, probably if we specify a config file (and it's not found) that should be an error as well.

[yuvipanda]

Closing as dupe of #73

[jbusecke]

Is this fully a dupe? This should also error out if no target/cache filesystem is in an (existing) config?

[yuvipanda]

@jbusecke yeah that's fair, re-opening