tests: install: Abort when one installation step fails.

When marionette-eval calls fail in gui-test-program, the installation
continues which results in two scenarios:

- hang forever at the next marionette-eval call,

- keep going and start a broken installation, which is annoying because it
clears the terminal and hides the error.

Make sure that gui-test-program is exited with #f return code when one of the
marionette-eval calls fail.

* gnu/tests/install.scm (gui-test-program): Add a new macro
"marionette-eval*". Throw an exception when one on the marionette-eval calls
fail.