Add typed error for thread creation exhaustion

- and make sure the supervisor knows about it
- bikeshedding on the new X::Exhausted exception may start

src/core/Exception.pm

@@ -897,6 +897,16 @@ my class X::Trait::Scope is Exception {
 }
 my class X::Comp::Trait::Scope is X::Trait::Scope does X::Comp { };
 
+my class X::Exhausted is Exception {
+    has $.what;
+    has $.reason;
+    method message {
+        $.reason
+          ?? "Could not create another $.what because of: $.reason"
+          !! "Could not create another $.what"
+    }
+}
+
 my class X::OutOfRange is Exception {
     has $.what = 'Argument';
     has $.got = '<unknown>';

src/core/Thread.pm

@@ -18,6 +18,7 @@ my class Thread {
       Str()  :$!name         = "<anon>"
       --> Nil
     ) {
+        constant THREAD_ERROR = 'Could not create a new Thread: ';
         $!vm_thread := nqp::newthread(
             anon sub THREAD-ENTRY() {
                 my $*THREAD = self;
@@ -30,6 +31,16 @@ my class Thread {
                 code();
             },
             $!app_lifetime ?? 1 !! 0);
+        CATCH {
+            when X::AdHoc {
+                .payload.starts-with(THREAD_ERROR)
+                  ?? X::Exhausted.new(
+                       :what<thread>,
+                       :reason(.payload.substr(THREAD_ERROR.chars))
+                     ).throw
+                  !! .rethrow
+            }
+        }
     }
 
     method start(Thread:U: &code, *%adverbs) {

src/core/ThreadPoolScheduler.pm

@@ -535,7 +535,8 @@ my class ThreadPoolScheduler does Scheduler {
 
                     # exhausted the system allotment of low level threads
                     if $!exhausted {
-                        $!exhausted = 0 if ++$!exhausted > EXHAUSTED_RETRY_AFTER;
+                        $!exhausted = 0  # try again in next run of supervisor
+                          if ++$!exhausted > EXHAUSTED_RETRY_AFTER;
                     }
 
                     # we can still add threads if necessary
@@ -555,14 +556,10 @@ my class ThreadPoolScheduler does Scheduler {
                       if $!affinity-workers.DEFINITE;
 
                     CATCH {
-                        when X::AdHoc {
-                            if .payload.starts-with("Could not create a new Thread") {
-                                $!exhausted = 1;
-                                scheduler-debug "Refraining from trying to start new threads";
-                            }
-                            else {
-                                scheduler-debug .gist;
-                            }
+                        when X::Exhausted {
+                            $!exhausted = 1;
+                            scheduler-debug .message;
+                            scheduler-debug "Refraining from trying to start new threads";
                         }
                         default {
                             scheduler-debug .gist;