pull: Add error prefixing with specific object when parsing

pull: Add error prefixing with specific object when parsing

One OpenShift user saw this from rpm-ostree:
```
client(id:cli dbus:1.583 unit:machine-config-daemon-host.service uid:0) added; new total=1
Initiated txn UpdateDeployment for client(id:cli dbus:1.583 unit:machine-config-daemon-host.service uid:0): /org/projectatomic/rpmostree1/rhcos
Txn UpdateDeployment on /org/projectatomic/rpmostree1/rhcos failed: File header size 4294967295 exceeds size 0
```

which isn't very helpful.  Let's add some error
prefixing here which would at least tell us which
object was corrupted.