SyncEngine: Abort sync with error if system exclude file is missing.

Syncing without the system exclude file would cause lots of trouble.
For instance we would try uploading the temporary files we store
partially completed downloads in.
SyncEngine: Fix invalid slot name in sync error condition.
SyncEngine: Use exclude file fallback next to binary on linux.

This is mainly useful for developers where the system-wide exclude file
may not exist.