Check downburst paths. Display an appropriate error message if an executable downburst cannot be found.
Check downburst paths. Display an appropriate error message if an
executable downburst cannot be found.

Fixes: #4693
Signed-off-by: Warren Usui <021fe6d8989d7bdb08c8700d8c426ea80a140097@inktank.com>
Reviewed by: Dan Mick <dan.mick@inktank.com>

dmick/teuthology,yghannam/teuthology,ceph/teuthology,dmick/teuthology,dmick/teuthology,michaelsevilla/teuthology,yghannam/teuthology,dreamhost/teuthology,ivotron/teuthology,robbat2/teuthology,caibo2014/teuthology,SUSE/teuthology,tchaikov/teuthology,ivotron/teuthology,ceph/teuthology,SUSE/teuthology,t-miyamae/teuthology,caibo2014/teuthology,dreamhost/teuthology,zhouyuan/teuthology,robbat2/teuthology,tchaikov/teuthology,t-miyamae/teuthology,michaelsevilla/teuthology,ktdreyer/teuthology,zhouyuan/teuthology,SUSE/teuthology,ktdreyer/teuthology