Suppress warning when logging is turned off

There was a case where a VTK-m exception was caught and then used in a
logging message. That is fine, but if VTK-m logging is turned off, that
code is removed and the exception variable goes unused.

There was a time when it was common to have logging turned off in VTK-m,
but now it is almost always on. There is one old-style dashboard that
has logging turned off and is giving a warning about this variable not
being used. It went unnoticed because the dashboard was down at the
time. This should fix the issue.
5 jobs for warning-no-logging in 0 seconds
Status Name Job ID Coverage
  External
passed cdash-buildbot

passed vtk-m-dragnipur-osx-shared-debug+32bit_ids+64bit_coords+clang+tbb

00:02:57

passed vtk-m-dragnipur-osx-static-release+64bit_coords+clang+tbb

00:04:12

passed vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb

00:24:23

passed vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb

00:32:45