Use .__str__ rather than .message to print errors.

[jorgenpt]

This helps because IOError does not assign `message', but str prints both the path and the error that occurred.

Old behavior:

$ bin/1pass foo
1pass: Error: 

New behavior:

$ bin/1pass foo               
1pass: Error: [Errno 2] No such file or directory: '/home/jorgen/Dropbox/1Password.agilekeychain/data/default/encryptionKeys.js'

[jorgenpt]

Fixes issue #7.

[AlJohri]

@AdamIsrael LGTM 👍