Also catch FileNotFoundError when detecting multiarch

[cottsay]

When the executable we're trying to invoke doesn't exist at all, we'll see a FileNotFoundError and not a subprocess.CalledProcessError.

Fixes #130

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 6.92%. Comparing base (c5452db) to head (a2d7b1c).
Report is 1 commits behind head on master.

Files	Patch %	Lines
colcon_ros/task/cmake/__init__.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #159   +/-   ##
======================================
  Coverage    6.92%   6.92%           
======================================
  Files          20      20           
  Lines         650     650           
  Branches      139     139           
======================================
  Hits           45      45           
  Misses        605     605           

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[nuclearsandwich]

Nice catch..

Except it's not called catching in Python. Oh well I tried.