[contrib] validate_failures.py: Use exit code "2" to indicate regression

[contrib] validate_failures.py: Use exit code "2" to indicate regression

... in the results.  Python exits with code "1" on exceptions and
internal errors, which we use to detect failure to parse results.

contrib/ChangeLog:

	* testsuite-management/validate_failures.py (Main): Use exit code "2"
	to indicate regression.