[bug] Failing examples do not make the build fail

This green build in AppVeyor: https://ci.appveyor.com/project/ConanOrgCI/examples/builds/30689171, the 2019 build output contains errors in the `waf` example:

```
[00:01:59] WafGen/0.1@user/channel: Already installed!
[00:01:59] mylib-waf/1.0@user/channel: WARN: Package is corrupted, removing folder: C:\Users\appveyor\AppData\Local\Temp\1\e513\.conan\data\mylib-waf\1.0\user\channel\package\3fb49604f9c2f729b85ba3115852006824e72cab
[00:01:59] mylib-waf/1.0@user/channel: WARN: Can't find a 'mylib-waf/1.0@user/channel' package for the specified settings, options and dependencies:
[00:01:59] - Settings: arch=x86_64, build_type=Release, compiler=Visual Studio, compiler.runtime=MD, compiler.version=16, os=Windows
[00:01:59] - Options:
[00:01:59] - Dependencies:
[00:01:59] - Package ID: 3fb49604f9c2f729b85ba3115852006824e72cab
[00:01:59]
[00:01:59] ERROR: Missing prebuilt package for 'mylib-waf/1.0@user/channel'
[00:01:59] Try to build it from sources with "--build mylib-waf"
[00:01:59] Or read "http://docs.conan.io/en/latest/faq/troubleshooting.html#error-missing-prebuilt-package"
[00:01:59]
[00:01:59]
[00:01:59] C:\projects\examples\features\integrate_build_system\mylib-consumer>conan build . --build-folder=build
[00:02:00] ERROR: conanbuildinfo.txt file not found in C:\projects\examples\features\integrate_build_system\mylib-consumer\build
[00:02:00] It is required for this command
[00:02:00] You can generate it using 'conan install'
```

or

```
[00:01:57] Waf: Entering directory `C:\Users\appveyor\AppData\Local\Temp\1\e513\.conan\data\mylib-waf\1.0\user\channel\build\3fb49604f9c2f729b85ba3115852006824e72cab\build'
[00:01:57] [1/2] Compiling src\mylib.cpp
[00:01:57] clang++: error: no such file or directory: '/MD'
[00:01:57] clang++: error: no such file or directory: '/EHsc'
[00:01:57] clang++: error: no such file or directory: '/O2'
[00:01:57] clang++: error: no such file or directory: '/Ob1'
[00:01:57] clang++: error: no such file or directory: '/DNDEBUG'
[00:01:57]
[00:01:57] Waf: Leaving directory `C:\Users\appveyor\AppData\Local\Temp\1\e513\.conan\data\mylib-waf\1.0\user\channel\build\3fb49604f9c2f729b85ba3115852006824e72cab\build'
[00:01:57] Build failed
```

Are we handling properly the errors from the `build.sh`, `build.bat` and `build.py` files we are executing? Do we need to check the return code in every step in `build.bat` and `build.sh` scripts?