Added better output on failure of length assertions.
Added better output on failure of length assertions.

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