We renamed caddytls.ErrStorageNotFound to caddytls.ErrNotExist to more
closely mirror the os package. We changed it to an interface wrapper
so that the custom error message can be preserved. Returning only "data
not found" was useless in debugging because we couldn't know the
concrete value of the error (like what it was trying to load).

Users can do a type assertion to determine if the error value is a "not
found" error instead of doing an equality check.