Don't expect .numerator to be set in DivByZero exceptions.

BDFL's blessing: http://colabti.org/irclogger/irclogger_log/perl6-dev?date=2018-09-21#l248

We no longer carry that information. The 420 test was also wrong from the
start as it used `isa-ok` instead of just `is`.

The test in question specs a tiny portion of the behaviour of
ZDRs (Zero Denominator Rationals), and the spec does not concern itself with
any of the other behaviours. Some of those turned out to be buggy and were
fixed. Others, are still buggy and need fixing. While a certain fix is
possible while maintaing this test in passing state, that fix inflicts a
non-trivial performance penalty on many operations on *all* Rationals.

Therefore, I argue the 6.c spec is wrong to spec the behaviour of ZDRs in
such a sparse manner, without any regard to how an implementation can
meet the goals of this particular test, while maintaining bug-free behaviour
of ZDRs. Ergo, the test is wrong and shall be removed.

S03-operators/arith.t

@@ -337,16 +337,13 @@ All uses of a zero modulus or divisor should 'die', and the
         'Division by zero with infix:<div> dies and is catchable with VRef variables';
 
     throws-like { say 0 / 0 }, X::Numeric::DivideByZero,
-        numerator => 0,
         'Division by zero with infix:</> dies and is catchable (1)';
     throws-like { say 3 / 0 }, X::Numeric::DivideByZero,
-        numerator => 3,
         'Division by zero with infix:</> dies and is catchable (2)';
     throws-like { my $x = 0; say 3.5 / $x }, X::Numeric::DivideByZero,
 #        numerator => 3.5,   # numerator is always an Int, so we get 7
         'Division by zero with infix:</> dies and is catchable with VInt/VRat variables';
     throws-like { my $x = 0; say 4 / $x }, X::Numeric::DivideByZero,
-        numerator => 4,
         'Division by zero with infix:</> dies and is catchable with VRef variables';
 }
 

S32-num/rat.t

@@ -331,8 +331,7 @@ is 241025348275725.3352.Str, "241025348275725.3352", 'stringification of bigish
 }
 
 #RT #126391
-try {say 42/(.1+.2-.3)};
-isa-ok( $!.numerator, 42, "got the answer rather than 420");
+try {say 42/(.1+.2-.3)}; isnt $!.numerator, 420, "no bogus errors";
 
 # RT#126016
 subtest '0.9999999999999999999999 to string conversions' => {