Handle is_warm exception

[andrewkrug]

If fs is read-only handle and throw exception that is_warm can not be tested.

[joelferrier]

Hey Jeff, would it be handy if I included a Dockerfile and some tooling for testing with a read-only filesystem locally?

[jeffbryner]

thanks @joelferrier I think I got it sorted, just made a readonly portion of my local system and tested write/non write conditions.