Fix showing error messages

Nate Grahamrequested to merge
ngraham/sddm-kcm:untranslatedmessage into master

In main.qml,message was renamed to untranslatedMessage with d6d13d38, but only in 2 out of the 3 places where it's used.

BUG: 471040 FIXED-IN: 5.27.6

@nicolasfella

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