lib: avoid REPL exit on completion error

If a tab completion is attempted on an undefined reference inside of a
function, the REPL was exiting without reporting an error or anything
else. This change results in the REPL reporting the ReferenceError and
continuing.

Fixes: https://github.com/nodejs/node/issues/3346
PR-URL: https://github.com/nodejs/node/pull/3358


Reviewed-By: James M Snell <jasnell@gmail.com>