shell: displays password error message

When a types in his password twice to confirm it is the desired password
an error message is now displayed under the text box. The text box
itself is now highlighted to further show this error.

Fixes: #739
The ```.check-password``` selector is too broad: This will add the class to all elements with that class, including the ones in #account-set-password-dialog.  This is harmless in this particular case (since only one of the dialogs will be visible at any given time), but might lead to surprises when we use this class in more places.