Only capture errors when clang return value is not zero.

Since stderr is parsed to warnings.html, we only capture stderr when
clang return value is not zero.
Bug: http://b/62292583
Test: python test_compiler.py ../../.. --build-only --target
aosp_angler-eng

Change-Id: If737a034b4f13a648df8cab26e704da2b9b695ff
2 files changed