Mogrify adverb slice logic into an Exception

So that we can properly test for them.  This should probably be
run at compile time somehow, but for now runtime will do.

src/core/Any.pm

@@ -1,6 +1,7 @@
 my class MapIter                { ... }
 my class Pair                   { ... }
 my class Range                  { ... }
+my class X::Adverb::Slice       { ... }
 my class X::Bind                { ... }
 my class X::Bind::Slice         { ... }
 my class X::Bind::ZenSlice      { ... }
@@ -830,8 +831,6 @@ sub RWPAIR(\k, \v) {   # internal fast pair creation
     p
 }
 
-sub OBJECT_HUH(\SELF) { try { SELF.VAR.name } // SELF.WHAT.perl }
-
 sub SLICE_HUH(\SELF, @nogo, %d, %adv) {
     @nogo.unshift('delete')  # recover any :delete if necessary
       if @nogo && @nogo[0] ne 'delete' && %adv.EXISTS-KEY('delete');
@@ -842,15 +841,11 @@ sub SLICE_HUH(\SELF, @nogo, %d, %adv) {
         }
     }
 
-    if %d.elems -> $elems {
-        $elems > 1
-          ?? fail "$elems unexpected named arguments (%d.keys.join(', ')) passed to {OBJECT_HUH(SELF)}"
-          !! fail "Unexpected named argument '%d.keys.[0]' passed to {OBJECT_HUH(SELF)}";
-    }
-
-    else {
-        fail "Unsupported combination of named arguments (@nogo.join(', ')) passed to {OBJECT_HUH(SELF)}";
-    }
+    fail X::Adverb::Slice.new(
+      :what(try { SELF.VAR.name } // SELF.WHAT.perl),
+      :unexpected(%d.keys.sort),
+      :@nogo,
+    );
 } #SLICE_HUH
 
 sub DELETEKEY(Mu \d, str $key) {

src/core/Exception.pm

@@ -926,6 +926,25 @@ my class X::Method::Private::Unqualified does X::Comp {
     }
 }
 
+my class X::Adverb::Slice is Exception {
+    has $.what;
+    has @.unexpected;
+    has @.nogo;
+    method message {
+        my $text = '';
+        if @!unexpected.elems -> $elems {
+            $text = $elems > 1
+              ?? "$elems unexpected adverbs (@!unexpected[])"
+              !! "Unexpected adverb '@!unexpected[0]'"
+        }
+        if @!nogo {
+            $text ~= $text ?? " and u" !! "U";
+            $text ~= "nsupported combination of adverbs (@!nogo[])";
+        }
+        $text ~ " passed to slice on $!what";
+    }
+}
+
 my class X::Bind is Exception {
     has $.target;
     method message() {