db: Gave each DatabaseClient class an 'executable_name' attribute (e.g.,...

db: Gave each DatabaseClient class an 'executable_name' attribute (e.g., 'psql' or 'mysql'), so that we can use it to make a more helpful error message. Refs #8978

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8989 bcc190cf-cafb-0310-a4f2-bffc1f526a37