docs - Set pbr 'warnerrors' option for doc build

By setting this pbr option in setup.cfg, the doc build will fail in case
of any warnings or errors occur during the build process.

Closes-Bug: #1351350

Change-Id: Id4858062d2aaa4c2fe5b597e40e4e8947f544a4d
Merge "docs - Set pbr 'warnerrors' option for doc build"