ament_clang_tidy - Fix Reporting when WarningsAsErrors is specified in config (backport #397)

[mergify[bot]]

Issue Summary

When WarningsAsErrors is specified in .clang-tidy, this can lead to some failures in result reporting. Specifically, the following test program (src/factorial.cpp):

int Factorial(int n) {
    int result = 1;
    int fake = 1;
    for (int i = 1; i <= n; i++) {
        result *= i;
    }
    return result;
}

fails with this output:

The invocation of "clang-tidy" failed with error code 1: Command '['/usr/bin/clang-tidy', '-p', 'path/test_examples_cmake', '--header-filter', 'include/test_examples_cmake/.*', 'path']' returned non-zero exit status 1.

when running ament_clang_tidy.

After applying this fix, I get the following output, which is expected

src/factorial.cpp:36:9: error: unused variable 'fake' [clang-diagnostic-unused-variable,-warnings-as-errors]
    int fake = 1;
        ^

Relevant section of .clang-tidy, for reference:

Checks:
    'bugprone*,
     clang-analyzer*,
     cert*,
     performance*,
     portability*,
     readability*,
     hicpp*,
     -hicpp-signed-bitwise,
     google-readability-todo,
    '
FormatStyle:     file
WarningsAsErrors: '*'

Fix

By trying to access the output attribute of the raised exception, we can populate the output in cases where exceptions were raised and there was valid output.

---

This is an automatic backport of pull request #397 done by Mergify.

[sloretz]

Assigned @clalancette to run CI and merge 🧇

[clalancette]

I'm going to suggest we hold off on this one until after the Jazzy release. It is low-risk, but also low-impact, so I think it is fine to put it in for a patch release.