Standardize exit statuses

[dag]

run() should do something more programmatic such as raise different exceptions depending on the outcome of the test run, if it had failures. It should not explicitly exit or raise SystemExit.

main() should be sensitive to the API of run() and translate to SystemExits.

Reporters should not handle exiting, but may need the ability to suppress exits for e.g. GUI reporters.

[dag]

• KeyboardInterrupt should probably exit with 130
• If tests failed (but nothing else did) exit status 1 is probably most compatible with existing Attest and other similar projects
• Exiting with status 2 seems de facto standard with optparse/argparse
• There's also sysexits.h with some conventions