Exit code of CONAN_PIP_INSTALL commands are not checked

[chausner-audeering]

Description of Problem, Request, or Question

When pip packages are specified in CONAN_PIP_INSTALL and the pip installation fails for some reason (e.g. because the package cannot be found or the pip remote is not reachable), cpt continues with the build and in my case fails at a later stage due to the missing packages.

I suggest to add a check for the exit code of the pip install and abort the build early if the package installation was not successful. This makes it easier to find out why builds fail when the pip installation is unsuccessful.

Environment Details

• Conan Package Tools Version: 0.35.1
• Operating System: Linux
• Operation System Version: Ubuntu 18.04
• Compiler+version: gcc-7
• Docker image: conanio/gcc7
• Conan version: conan 1.35.1
• Python version: python 3.6.9