PERL-336 don't throw exceptions on command queries

Prior to this commit, the Cursor.xs code threw an exception if it saw
'$err' in a document.  This prevents command responses from being
returned, which prevents inspection of the code returned with the
error, if any.

This commit only throws on '$err' for collections other than '$cmd'.
skip parallel scan on mongos
wip: XXX probe test failure
wip: another test probe