Don't mangle precondition errors.
Don't mangle precondition errors.

mbrukman/gcsfuse,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,jacobsa/timeutil,jacobsa/ratelimit,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,kahing/gcsfuse