Add message to NotSquareException thrown from Avatar

This prevents cryptic messages such as the following, from `user_ldap`:

     Could not set avatar for uid=user,ou=People,dc=example,dc=net, because:

Signed-off-by: Olivier Mehani <shtrom@ssji.net>

Add message to NotPermittedException thrown from Files\Nodes\Folder

Ditto.

Don't use translation macros here as this seems to be pretty low-level
errors that generally get caught and prettified, and I don't want to
unduly clog down the lower layers.

Signed-off-by: Olivier Mehani <shtrom@ssji.net>

fixup! Add message to NotPermittedException thrown from Files\Nodes\Folder