GitX quits when its history cannot be loaded #645

PBGitRevList.m:264 handles a failed commit walk with an assertion:

while ((oid = [enumerator nextOIDWithSuccess:&enumSuccess error:&enumError]) && enumSuccess && !operation.cancelled) {
	...
}

NSAssert(!enumError, @"Error enumerating commits");

So any enumeration error raises NSInternalInconsistencyException instead of
being reported. Reachable whenever the repository changes under an open window:
deleted or moved, a network volume dropping out, a corrupt object, permissions
changing.

This is not Debug-only. NS_BLOCK_ASSERTIONS is not set for Release, and the
message is present in shipped binaries:

$ strings /Applications/GitX.app/Contents/MacOS/GitX | grep -c "Error enumerating commits"
1

enumError is already populated, so it can be surfaced the way other failures
in this class are.

Found while fixing #643, where deleting a test fixture under a running walk hit
this assertion. That PR stops the tests provoking it; the handling here is
unchanged.