Detect valid pathnames before checking whether a path exists (fixes #45)

[batterseapower]

The easiest fix for #45 is just to catch the ValueError, but this is a more comprehensive solution that should handle the general case where a string is supplied that is not also a valid path name.

[coveralls]

Coverage decreased (-4.5%) to 31.061% when pulling 0a3830a on batterseapower:master into a18c171 on stchris:master.

[stchris]

I'm hoping to fix this in #87 by separating the file lookup logic in the input auto-detection of the parse function.