Use const references when catching exceptions

[darbyjohnston]

Fixes #1033

This PR fixes a couple warnings from gcc 10.3:

warning: catching polymorphic type ‘class std::exception’ by value [-Wcatch-value=]

[codecov-commenter]

Codecov Report

Merging #1034 (bd7f66e) into master (a84265c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1034   +/-   ##
=======================================
  Coverage   79.30%   79.30%           
=======================================
  Files         113      113           
  Lines        6102     6102           
=======================================
  Hits         4839     4839           
  Misses       1263     1263           

Flag	Coverage Δ
unittests	79.30% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/opentime/rationalTime.cpp	85.32% <ø> (ø)

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