Description of problem:

As CNV team suggested, nmstate could provide smart(with suggestions on which line to fix or even better suggesions) error message in these use cases:

A: User try to move eth1 to bridge and forgot to move routes also.
B: IP enabled but no IP address and no DHCP server.
C: Assigning eth1 to br0, but eth1 does not exist, there is a similar eth0 found.

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