Implement X::Numeric::CannotConvert exception

It's a generic exception to throw when a numeric cannot be
converted to something (gonna use it for Inf -> Int coercion).
Ideally, X::Numeric::Real would be removed and this exception
be used in its place, but there are 6.c-errata tests expecting
X::Numeric::Real to exist, so I left it in as just an empty
subclass of X::Numeric::CannotConvert

src/core/Exception.pm

@@ -2404,15 +2404,17 @@ my class X::Import::Positional is Exception {
     }
 }
 
-my class X::Numeric::Real is Exception {
+my class X::Numeric::CannotConvert is Exception {
     has $.target;
     has $.reason;
     has $.source;
 
     method message() {
         "Cannot convert $.source to {$.target.^name}: $.reason";
     }
+
 }
+my class X::Numeric::Real is X::Numeric::CannotConvert {}
 
 my class X::Numeric::DivideByZero is Exception {
     has $.using;