libmm-glib,simple-connect-properties: cleaner error handling

If processing a key-value pair as a bearer property fails, we need to
know if it failed due to the key being unknown or due to some other
reason (e.g. failure parsing value of a known key).

We'll only try with the Simple.Connect properties if the key is
reported as unknown in the bearer properties.

This will help us better identify errors if e.g. an invalid value is
given to a known key. E.g. "yes" was invalid for allow-roaming here:

    daemon.notice netifd: wan (30476): simple connect=apn=internet,ip-type=ipv4,allow-roaming=yes
    daemon.notice netifd: wan (30476): Error parsing connect string: 'Invalid properties string, unexpected key 'allow-roaming''

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