fix: extraction of module UI

[DavidePrincipi]

The cluster leader keeps a copy of UI code. During the module update the previous UI code is renamed before being deleted. If an error occurs it is ignored in this case.

Mar  5 07:37:55 server1 agent@cluster[2049224]: Module UI update failed for instance nethvoice-proxy2: [Errno 2] No such file or directory: '/var/lib/nethserver/cluster/ui/apps/nethvoice-proxy2' -> '/var/lib/nethserver/cluster/ui/apps/nethvoice-proxy2.d1bf94bf~'
Mar  5 07:37:55 server1 agent@cluster[2049224]: Assertion failed
Mar  5 07:37:55 server1 agent@cluster[2049224]:  File "/var/lib/nethserver/cluster/actions/update-module/50update", line 154, in <module>
Mar  5 07:37:55 server1 agent@cluster[2049224]:    agent.assert_exp(errors == 0)
Mar  5 07:37:55 server1 agent@cluster[2049224]: task/cluster/d1bf94bf-7cb6-4250-a54f-7338e4ab1a7f: action "update-module" status is "aborted" (2) at step 50update

Refs https://mattermost.nethesis.it/nethesis/pl/utjk4jt117r5jr9z14oou9qu6y