Add an X::MOP; make an existing exception do it.

We'll use this to flag errors from the MOP that are to be displayed
verbatim, rather than as if they were unexpected due to some internal
MOP failure.

src/core/Exception.pm

@@ -448,6 +448,8 @@ my class X::Comp::Group is Exception {
     }
 }
 
+my role X::MOP is Exception { }
+
 # XXX a hack for getting line numbers from exceptions from the metamodel
 my class X::Comp::AdHoc is X::AdHoc does X::Comp {
     method is-compile-time() { True }
@@ -1653,7 +1655,7 @@ my class X::Caller::NotDynamic is Exception {
     }
 }
 
-my class X::Inheritance::NotComposed is Exception {
+my class X::Inheritance::NotComposed does X::MOP {
     # normally, we try very hard to capture the types
     # and not just their names. But in this case, both types
     # involved aren't composed yet, so they basically aren't