Fix for issue #1 - return unexpected errors early

[laher]

this behaviour could potentially break someone's use of the library...

e.g. if they have one unreliable filesytem, this error might prevent you from ever reaching the correct filesystem.

Edit: I have provided one workaround:

• mergefs.MergeWithOptions(mergefs.AllowError(my.TimeoutError))
• If you want to permit certain errors, you can initialise mergefs with that kind of support.

It still doesn't feel quite right though. I'll think on it some more