Add error message on failing to create config

[cforfang]

Hi, it took me a while to realize why I couldn't create a reverse mapping from a folder where I only have read permissions because no error is printed:

$ mkdir test0 test1
$ chmod -w test0
$ encfs --reverse $(pwd)/test0 $(pwd)/test1
[...]
New Encfs Password: 
Verify Encfs Password: 
$

This commit adds one:

$ encfs --reverse $(pwd)/test0 $(pwd)/test1
[...]
New Encfs Password: 
Verify Encfs Password: 
Error saving config file.
$

[vgough]

Thanks