main: check if the config file can opened before prompting for password
main: check if the config file can opened before prompting for password

This was frustrating:

$ gocryptfs a b
Password:
Decrypting master key
open a/gocryptfs.conf: permission denied

rfjakob/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs