Fixed #8978 -- We now print a helpful error message for 'manage.py dbshell' if...

Fixed #8978 -- We now print a helpful error message for 'manage.py dbshell' if the client executable is not found. Previously we were displaying a traceback

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8990 bcc190cf-cafb-0310-a4f2-bffc1f526a37