js-lint doesn't crash if console.log isn't in file

`grep` returns a non-zero exit code when the pattern isn't in the file,
and `set -e` causes the script to exit when a non-zero exit code
happens. For the duration of the `grep`, we need to `set +e / set -e` to
prevent the script from exiting prematurely.

This (and all non-Ruby) hooks should be converted to Ruby because we're
probably all better Ruby devs than Bash devs, and this might have been
more obvious when it was written.