Merge from widevine repo of http://go/wvgerrit/17202
This allows more checks in the test to run. Returning the correct
error code is less important than the next check, which verifies the
signature was not computed.
This helps diagnose b/21708882, but does not fix it.
b/27787064
Change-Id: I65306c54b77370951bc54e8ef9f6140f58bea32c