src: don't error at startup when cwd doesn't exist

The current working directory may not exist when iojs starts up. Don't treat that as an error because it's still possible to do many useful things, like evaluating a command line script or starting a REPL.

Fixes: #1184 (closed)

R=@mscdex?

https://jenkins-iojs.nodesource.com/view/iojs/job/iojs+any-pr+multi/326/