Noticed that tox config -e py47 returns exit code 0 even though the output is full of exception markers for the missing interpreter. Meanwhile tox -e py47 run correctly returns 255.

This updates the config command to return exit code -1 (255) when any config values raise exceptions during display, making it consistent with how run handles the same scenario.

Before:

$ tox -e py47 config
[testenv:py47]
...
env_site_packages_dir = # Exception: RuntimeError(...)
...
$ echo $?
0

After:

$ tox -e py47 config
[testenv:py47]
...
env_site_packages_dir = # Exception: RuntimeError(...)
...
$ echo $?
255

The change is minimal -- print_conf now returns a boolean indicating whether any exceptions occurred, and show_config uses that to decide the exit code.

Fixes #3649