Default high precision off if the mtime call fails

This is a bit of an edge case. When using the gem under JRuby inside a JAR on Windows, the mtime call fails with SystemCallError unknown error 20047.

I assume that this is because it's run under a JAR archive and Windows does something different from OSX that makes the mtime call fail. It works fine under OSX, and I imagine Linux as well.

Since it's a bit of an edge case, and the alternative I see is to hook it into the listen test in `works?`, I figured it's better to just default to high precision not being available if it errors.