Die early for a bad --temp_dir setting.
Die early for a bad --temp_dir setting.

Fixes #49.

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