fix cases where a missed cb() would result in manta-sync stopping with unfinished work, exiting 0 when there were errors

[trentm]

a failing example run: https://gist.github.com/trentm/7ac546372af934cb30859ba6cd87a3dd

a working example run: https://gist.github.com/trentm/960be6afb7f3697d7f5df45d484cfb41 (where "working" means that manta-sync properly exited non-zero when there were auth errors).

The repro case was when doing a large manta-sync (from manta to local) very soon after adding the used SSH key to the account.

[bahamas10]

good catch, thanks!

[bahamas10]

$ npm publish
+ manta-sync@0.3.4