Added additional error checking to FileWatch::Tail#_open_file

[nealpatel-work]

I fixed a bug I encountered.

FileWatch tried to stat files located in directories without the +x flag. This caused FileWatch to raise an error (Errno::EACCES), which it shouldn't do (regardless of what permissions are set on the file).

[jsvd]

I agree that this is a poor error message, but not sure if adding the string "or stat" helps users understand that the problem comes from the directory's permissions.
maybe, on exception, check them?

begin
   # ...
rescue
   # ..
   dir = File.dirname("dir/file")
   unless File.executable?(dir) && File.writable?(dir) do
     @logger.error "Directory has permission issues #{File.stat(dir).inspect}"
   end
 end