Do not report CVSDIFF when files are missing
Do not report CVSDIFF when files are missing

When a gold file or a result file was missing, CSVDiff reported a diff
status. That is confusing, thus now we report the actual problem.

Refs #1877

jessecarterMOOSE/moose,laagesen/moose,YaqiWang/moose,bwspenc/moose,lindsayad/moose,milljm/moose,lindsayad/moose,bwspenc/moose,milljm/moose,permcody/moose,laagesen/moose,sapitts/moose,sapitts/moose,laagesen/moose,jessecarterMOOSE/moose,jessecarterMOOSE/moose,dschwen/moose,harterj/moose,idaholab/moose,SudiptaBiswas/moose,milljm/moose,dschwen/moose,jessecarterMOOSE/moose,dschwen/moose,friedmud/moose,lindsayad/moose,permcody/moose,sapitts/moose,sapitts/moose,idaholab/moose,idaholab/moose,harterj/moose,harterj/moose,dschwen/moose,idaholab/moose,andrsd/moose,nuclear-wizard/moose,jessecarterMOOSE/moose,andrsd/moose,andrsd/moose,milljm/moose,harterj/moose,friedmud/moose,idaholab/moose,bwspenc/moose,milljm/moose,laagesen/moose,nuclear-wizard/moose,SudiptaBiswas/moose,nuclear-wizard/moose,bwspenc/moose,nuclear-wizard/moose,YaqiWang/moose,lindsayad/moose,laagesen/moose,YaqiWang/moose,permcody/moose,andrsd/moose,sapitts/moose,SudiptaBiswas/moose,dschwen/moose,harterj/moose,friedmud/moose,friedmud/moose,bwspenc/moose,permcody/moose,andrsd/moose,YaqiWang/moose,SudiptaBiswas/moose,lindsayad/moose,SudiptaBiswas/moose