the #NMConnection object did not contain the required #NMSettingConnection object, which must be present for all connections
the 'type' property of the 'connection' setting did not point to a valid connection base type; ie it was not a hardware-related setting like #NMSettingWired or #NMSettingWireless.
the #NMConnection object contains a conflicting setting object
the #NMConnection object did not contain the specified #NMSetting object
unknown or unclassified error
Describes errors that may result from operations involving a #NMConnection.