treat warnings as errors

When building docs with Travis

.travis.yml

@@ -43,7 +43,7 @@ script:
   - if [[ $BUILD_DOCS == false ]]; then mkdir ../tmp_test_dir; fi
   - if [[ $BUILD_DOCS == false ]]; then cd ../tmp_test_dir; fi
   - if [[ $BUILD_DOCS == false ]]; then gdb -return-child-result -batch -ex r -ex bt --args python ../matplotlib/tests.py -sv --processes=8 --process-timeout=300 $TEST_ARGS; fi
-  - if [[ $BUILD_DOCS == true ]]; then cd doc; python make.py html --small; fi
+  - if [[ $BUILD_DOCS == true ]]; then cd doc; python make.py html --small --warningsaserrors; fi
   # We don't build the LaTeX docs here, so linkchecker will complain
   - if [[ $BUILD_DOCS == true ]]; then touch build/html/Matplotlib.pdf; fi
   - if [[ $BUILD_DOCS == true ]]; then linkchecker build/html/index.html; fi