Renamed exception to X::Promise::Resolved

src/core.c/Promise.pm6

@@ -15,10 +15,10 @@ my class X::Promise::Vowed is Exception {
     has $.promise;
     method message() { "Access denied to keep/break this Promise; already vowed" }
 }
-my class X::Promise::IllegalTransition is Exception {
+my class X::Promise::Resolved is Exception {
     has $.promise;
     method message() {
-        "Illegal attempt to keep/break this Promise (status: $!promise.status())";
+        "Cannot keep/break a Promise more than once (status: $!promise.status())";
     }
 }
 my role X::Promise::Broken {
@@ -102,7 +102,7 @@ my class Promise does Awaitable {
 
     method !keep(Mu \result --> Nil) {
         $!lock.protect({
-            X::Promise::IllegalTransition.new(promise => self).throw
+            X::Promise::Resolved.new(promise => self).throw
              if $!status != Planned;
 
             $!result := result;
@@ -134,7 +134,7 @@ my class Promise does Awaitable {
 
     method !break(\result --> Nil) {
         $!lock.protect({
-            X::Promise::IllegalTransition.new(promise => self).throw
+            X::Promise::Resolved.new(promise => self).throw
              if $!status != Planned;
 
             $!result := nqp::istype(result, Exception)