[Fix] `nvm_ls_remote_combined`: propagate iojs remote listing failures

`return $A || $B` only evaluates the first argument, since `return` always succeeds.
The io.js exit code was never checked, silently swallowing remote listing failures.

Bug introduced in https://github.com/nvm-sh/nvm/commit/ea127846290b1126556e256031383d171a4e38d8 / #616.