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 :warning:

Additional details and impacted files

```diff @@ 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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