Support a :&catch argument for start() and Promise.start()

This allows timely detection of exceptions thrown by tasks
that may not be await'ed for some time (e.g. because they
are background tasks not expected to complete for a long time),
and mirrors the same parameter on ThreadPoolScheduler.cue().

src/core/Promise.pm

@@ -155,12 +155,12 @@ my class Promise {
         }
     }
 
-    method start(Promise:U: &code, :$scheduler = $*SCHEDULER) {
+    method start(Promise:U: &code, :&catch, :$scheduler = $*SCHEDULER) {
         my $p   = Promise.new(:$scheduler);
         my $vow = $p.vow;
         $scheduler.cue(
             { $vow.keep(code()) },
-            :catch(-> $ex { $vow.break($ex) }) );
+            :catch(-> $ex { catch($ex) if &catch; $vow.break($ex); }) );
         $p
     }
 
@@ -202,6 +202,6 @@ my class Promise {
 
 # Schedules a piece of asynchronous work using the current scheduler, and
 # returns a Promise that represents it.
-sub start(&code) { Promise.start(&code) }
+sub start(&code, :&catch) { Promise.start(&code, :&catch) }
 
 # vim: ft=perl6 expandtab sw=4