Add CommandExecutionError to exception handling

aptpkg's set_selections (called by hold/unhold) could potentially raise
this exception, so catch it as well.
aptpkg: Fix call to get_selections()

This was passing "hold" as a name, not a quoted string (which succeeded
since there is a function named "hold" in the module), causing the
retrieved selections to be empty, leading to a traceback when the
function later tries to run set_selections().