settings: return errno from nms_keyfile_nmmeta_write() for better logging

I encountered a failure in the log

    <trace> [1564647990.7822] keyfile: commit: deleting nmmeta file "/etc/NetworkManager/system-connections/35370b0b-e53b-42ea-9fe3-f1b1d552343b.nmmeta" failed
    <trace> [1564647990.7822] keyfile: commit: deleting nmmeta file "/etc/NetworkManager/system-connections/35370b0b-e53b-42ea-9fe3-f1b1d552343b.nmmeta" simulated

I think that was due to SELinux (rh #1738010).

Let nms_keyfile_nmmeta_write() return an errno code so we can log
more information about the failure.
15 jobs for th/settings-improvements in 55 minutes and 19 seconds (queued for 1 minute and 10 seconds)
Status Job ID Name Coverage
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checkpatch

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manual #486128
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t_centos:7.5.1804
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t_centos:7.6.1810

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t_debian:10

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t_debian:testing
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t_fedora:rawhide
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t_ubuntu:18.04

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t_ubuntu:devel
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t_ubuntu:rolling