Test build should not proceed if virtualenv creation fails

Currently CMake/GkfsPythonTesting.cmake relies on the following code to install tests:

install(
    CODE "message(\"Install pytest virtual environment...\")"
    CODE "message(\"-- Create virtual environment: ${PYTEST_VIRTUALENV}\")"
    CODE "execute_process(COMMAND ${Python3_EXECUTABLE} -m venv ${PYTEST_VIRTUALENV})"
    CODE "message(\"-- Installing packages...\")"
    CODE "execute_process(COMMAND ${PYTEST_VIRTUALENV}/bin/pip install --upgrade pip -v)"
    CODE "execute_process(COMMAND ${PYTEST_VIRTUALENV}/bin/pip install -r ${CMAKE_CURRENT_BINARY_DIR}/requirements.txt --upgrade -v)"
)

This has two problems:

The user is not cleanly notified of an error in the CODE chain (e.g. when the virtualenv is created).
The installation proceeds even if a previous step failed (see below).

-- Install configuration: "Debug"
-- Installing: /home/amiranda/var/projects/gekkofs/prefix/bin/gkfs_daemon
-- Set runtime path of "/home/amiranda/var/projects/gekkofs/prefix/bin/gkfs_daemon" to "/home/amiranda/var/projects/gekkofs/prefix/lib"
-- Installing: /home/amiranda/var/projects/gekkofs/prefix/lib/libgkfs_intercept.so
-- Set runtime path of "/home/amiranda/var/projects/gekkofs/prefix/lib/libgkfs_intercept.so" to "/home/amiranda/var/projects/gekkofs/prefix/lib"
-- Installing: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/pytest.ini
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/conftest.py
Install pytest virtual environment...
-- Create virtual environment: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/pytest-venv
The virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venv
package using the following command.

    apt-get install python3-venv

You may need to use sudo with that command.  After installing the python3-venv
package, recreate your virtual environment.

Failing command: ['/home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/pytest-venv/bin/python3.8', '-Im', 'ensurepip', '--upgrade', '--default-pip']

-- Installing packages...
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/harness
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/harness/workspace.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/harness/io.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/harness/cli.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/harness/logger.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/harness/reporter.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/harness/cmd.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/harness/gkfs.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/directories
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/directories/test_directories.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/status
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/status/test_status.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/operations
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/operations/test_write_operations.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/operations/test_read_operations.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/position
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/position/test_lseek.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/data
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/data/test_truncate.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/data/test_data_integrity.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/shell
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/shell/test_concat.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/shell/test_cp.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/shell/test_stat.py
-- Up-to-date: /home/amiranda/var/projects/gekkofs/prefix/share/gkfs/tests/integration/shell/test_stat.py.bak
-- Installing: /home/amiranda/var/projects/gekkofs/prefix/bin/gkfs.io
-- Installing: /home/amiranda/var/projects/gekkofs/prefix/bin/tests