fakefs: All fail invocations need to coerce pathnames to strings

Behavior we expect to see: useful error messages, e.g. file doesn’t exist

Behavior we see:

    TypeError:
       no implicit conversion of FakeFS::Pathname into String

Where we see it: Basically anywhere that there is an invocation of fail that passes an object that could be a Pathname, which is a lot of places. These fail invocations are not collected in one location at all.