avocado.core.output: Fix incorrect exception handling in get_paginator()

The actual exception we're throwing in process.find_command
is process.CmdNotFoundError.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #89 from avocado-framework/fix-output-paginator

avocado.core.output: Fix incorrect exception handling in get_paginator()