Use custom cache over FSCache if specified (#1285)
* Use custom cache over FSCache if specified
Currently, if both `useFSCache` and `useCustomCache` env options are set to true (perhaps because `useFSCache` is true by default and the user forgot to unset it), the code defaults to instantiating a FileCache instead of using the custom cache implementation passed in by the user. This commit changes the default to be the custom cache if specified.
* Use custom cache over browser cache if specified
If `useCustomCache` is set, try to use the custom cache before falling back to the browser or the fs caches.
* Throw an error if fs is not available for FSCache
Meant to handle cases where the user sets `useFSCache` from an environment that doesn't support `fs`.
