broadband-modem-qmi: log reason of QMI port open failure

If ModemManager fails to open a QMI port with data format update, it
retries without update. It would be useful to log the reason why the
first try failed; for example on RHEL 7.6 ModemManager fails to write
to sysfs due to SELinux. With this commit, the following message would
be logged:

 <debug> QMI port open operation finished
 <debug> Couldn't open QMI port with data format update: Expected data
 format not updated properly: Failed to open file
 '/sys/class/net/wwp0s29u1u2i8/qmi/raw_ip' for R/W: Permission denied
 <debug> Opening QMI device...

instead of just:

 <debug> QMI port open operation finished
 <debug> Opening QMI device...

While at it, also fix the GError leak.

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