Fix a misleading error message.

I couldn't understand on https://issuetracker.google.com/273841150
why we saw a file truncation failure error message followed by a
claim that ExtractToMemory() had failed, when only ExtractToFile()
should be doing anything with files...

Change-Id: Ic3b2c8b390a7b9f7a2e5c8b229b55c46acf6638e