lib: don't error in repl when cwd doesn't exist

The current working directory may not exist when the REPL starts up.
Don't treat that as an error because it's still possible to do many
useful things.  This is like the previous commit but for the REPL.

Fixes: https://github.com/iojs/io.js/issues/1184
PR-URL: https://github.com/iojs/io.js/pull/1194


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rod Vagg <rod@vagg.org>