broadband-modem: report signal quality as 0 when no service/unknown/etc

Instead of using the NO_NETWORK error, which is only ever read
internally by ModemManager and not very useful when reading quality,
just report signal quality as zero.  Before MM 1.0, the signal quality
could be explicitly requested by clients, so the NO_NETWORK error was of
some use, but now that the quality is updated automatically by MM and is
a property instead of a D-Bus method call, the error is pointless.

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