`fetchAppIDArray()`: Ignore nonexistent ID errors.
Bring `EmceeStore` in line with `GomemcStore`.

* `{Emcee, No}Store.Exists()` now correctly returns `false` if a device
  ID has not previously registered with the push server.
* `EmceeStore.Exists()` should log other error types.
Update tests.

All tests now pass for the `memcache_gomc`, `memcache_memcachego`,
and `none` storage adapters.
Revert `GomemcStore.fetchAppIDArray()` change.
Merge pull request #65 from mozilla-services/feature/emcee-gomemc-parity

Bring `{Emcee, No}Store` in line with `GomemcStore`