ide: Improve error messages

Use error_report(), because it points to the error location.

Reword "tried to assign twice" messages to make it clear that we're
complaining about the unit property.

Report invalid unit property instead of failing silently.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
