Catch ProcessExecutionError in revoke_cert

Catch processExecutionError if revoking the
certificate fails.

This change has been abandoned by Chuck Short, so
I am continuing where he left off (mattoliverau).

Continues abandoned change: 17741

Change-Id: I9714ea8cece87256ff5f9a936286c1da3d628af9
Closes-Bug: #883320
Co-Authored-By: chuck.short@canonical.com
Merge "Catch ProcessExecutionError in revoke_cert"