"Transport endpoint is not connected" on multiple failed password attempt

[EvansJahja]

Possibly related to #67

What happened:
When trying to mount, putting wrong password multiple times or putting empty password on mounts that use idle,exptass,delaymount,ondemand causes "Transport endpoint is not connected", and I can no longer attempt to put in password.

What should happen:
Putting empty wrong passwords shouldn't cause error, and I should be able to try the passwords indefinitely.

Steps to reproduce:

1. mkdir a b
2. encfs ~/a ~/b -i 1 --extpass="dmenu" --delaymount -m
3. When prompted, I choose the standard encryption, and types "qweqwe" as the password.
4. encfs -u ~/b
5. repeat step (2)
6. type ls b causes dmenu to appear, asking for password.
   7a. If I enter blank, the issue suddenly arises. Go to (8)
   7b. If I enter wrong password like "asdf", typing ls b will shows dmenu and reprompt the passwords. After multiple attempts (like 3 times) the issue will arise. Go to (8)
7. typing ls b will yield

ls: cannot access 'b': Transport endpoint is not connected

No further password prompt is shown. cd b shows similar result.
9. To recover, we can encfs -u ~/b and continue from step (5), which will again cause the issue.

[EvansJahja]

Update: I think I've isolated the issue:

When I run encfs with --delaymount, the program will stay in background and waits until user access files/folders that encfs watches. When user access those files, like just using ls, encfs will know that it needs to unlock the folder/file and asks for password using extpass. However, If user enters empty string (by hitting enter) or cancels the action, the program would forcefully exit, because of this line. While this is okay if encfs is run interactively, it is very bad if the program just exits while it's running in the background because user enters empty string, whereas if user had entered invalid password, the program would continue to run, and prompting for user to retry the password.

Moving the exit(1) logic to the calling function (like main) should allow exit(1) to be not called when the program stay in the background. Alternatively, allowing empty password if we're dealing with folders/files that already exists (i.e. we're not creating / modifying a new folder, just accessing) should be allowed. Even better, we could just prevent that line from being called by checking for user input before we get there, allowing to reject the user input if it's empty.

Comments?