`make clean` and `sdw-admin --uninstall` halt execution on error

[emkll]

When running the clean makefile target [1] (or the associated prod securedrop-admin target [2]), the operation will stop as soon as an error occurs. This means that the workstation is in a potentially unpredictable state, and some cleanup operations may not complete.

For example, in the case of functionality added https://github.com/freedomofpress/securedrop-workstation/pull/533, the custom tags are removed after vms are destroyed, if a Template is not destroyed due to an error ( for example TemplateVM used by an unmanaged AppVM), the tags will not be removed.

Since it is inherently destructive operation we should consider ignoring errors at runtime and continue execution, and report errors at the end

[1] https://github.com/freedomofpress/securedrop-workstation/blob/079ff4ef98e1c1c8ca8b3caa414a0dfaba2205bc/Makefile#L110 [2] https://github.com/freedomofpress/securedrop-workstation/blob/079ff4ef98e1c1c8ca8b3caa414a0dfaba2205bc/scripts/securedrop-admin.py#L98