--extpass return code is not checked

[mserdarsanli]

For getting the user password EncFSConfig::getUserKey does not seem to check exit status of the external command and use its stdout regardless.

If --extpass is used while creating a new filesystem and the external program fails with some output on its stdout (otherwise you get fatal: zero-length passwords are not allowed ), then user would be creating a filesystem with potentially no way to recover files. I noticed this by passing wrong flags to external process that caused it to print usage text and fail, while filesystem creation succeeded. encfs command should instead check exit status of subprocess after the waitpid call and fail if it is not 0.