Call trace occurred after a wrong wireless interface mistakenly applied once in the yaml

[Vic Liu]

Call trace occurred every time when running `netplan apply` after a wrong wireless interface was once described in the yaml.

System: Intel NUC7PJYH
Image: Ubuntu Core 20 beta daily build (20200514.2)

Steps to Reproduce:
1. On this Intel NUC7PJYH I tested, its wireless interface is wlo2 instead of wlan0, I mistakenly described the wireless interface as wlan0 in the yaml at the first time, corrected it to wlo2 as far as I found the wireless connection is unreachable.

2. After the wireless interface was corrected to wlo2, following call trace happened every time running `netplan apply`:

$ sudo netplan apply
Failed to start netplan-wpa-wlan0.service: Unit netplan-wpa-wlan0.service not found.
Traceback (most recent call last):
  File "/usr/sbin/netplan", line 23, in <module>
    netplan.main()
  File "/usr/share/netplan/netplan/cli/core.py", line 50, in main
    self.run_command()
  File "/usr/share/netplan/netplan/cli/utils.py", line 179, in run_command
    self.func()
  File "/usr/share/netplan/netplan/cli/commands/apply.py", line 46, in run
    self.run_command()
  File "/usr/share/netplan/netplan/cli/utils.py", line 179, in run_command
    self.func()
  File "/usr/share/netplan/netplan/cli/commands/apply.py", line 173, in command_apply
    utils.systemctl_networkd('start', sync=sync, extra_services=netplan_wpa)
  File "/usr/share/netplan/netplan/cli/utils.py", line 86, in systemctl_networkd
    subprocess.check_call(command)
  File "/usr/lib/python3.8/subprocess.py", line 364, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['systemctl', 'start', '--no-block', 'systemd-networkd.service', 'netplan-wpa-wlo2.service', 'netplan-wpa-wlan0.service']' returned non-zero exit status 5.

3. When a call trace happened, the wireless could still connect to the AP without problem. When system connected to the AP described in yaml, it still has the ability get an IP from it and ping the gateway on AP.

[Lukas Märdian (slyon)]

I assume this got fixed in https://github.com/canonical/netplan/pull/133 especially https://github.com/canonical/netplan/pull/133/commits/f2ff1f4f7d8d1a41b7d6021b13b46d92e0736d0b as of Netplan v0.100

Please re-open if this is still an issue.