fix problem with specializing rigids

compiler/unify/src/unify.rs

@@ -681,7 +681,7 @@ fn unify_shared_tags(
 
         merge(subs, ctx, Structure(flat_type))
     } else {
-        mismatch!()
+        mismatch!("Problem with Tag Union")
     }
 }
 
@@ -911,7 +911,7 @@ fn unify_rigid(subs: &mut Subs, ctx: &Context, name: &Lowercase, other: &Content
         RigidVar(_) | Structure(_) | Alias(_, _, _) => {
             // Type mismatch! Rigid can only unify with flex, even if the
             // rigid names are the same.
-            mismatch!()
+            mismatch!("Rigid with {:?}", &other)
         }
         Error => {
             // Error propagates.