Fixes #15648
Functional tests can fail and lcov still generates a coverage file, which is inaccurate. This change stops make from proceeding if functional tests fail.
before:

after:

Concept ACK. Thanks for working on this!
straightforward enough ACK 9218ce8d48e504aeaa983e82b206a386712e43e4
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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.
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| Coverage | Change (pull 16207, 6417c291887bbde7f2374a77073988e55521a062) | Reference (master, 413e438ea9767710d4810c4400fd1242ca52cd1c) |
|---|---|---|
| Lines | -0.0278 % | 89.0544 % |
| Functions | -0.0314 % | 82.0481 % |
| Branches | -0.0244 % | 45.1906 % |
<sup>Updated at: 2019-06-20T20:11:39.530365.</sup>