Make backtrace printing more robust

src/core/Backtrace.pm

@@ -80,13 +80,13 @@ my class Backtrace is List {
         return $startidx.list unless $start;
         my $current = $start.outer;
         my %outers;
-        while $current {
+        while $current.defined {
             %outers{$current.static_id} = $start;
             $current = $current.outer;
         }
         my @outers;
         loop (my Int $i = $startidx; $i < $.end; ++$i) {
-            if self.at_pos($i).code && %outers{self.at_pos($i).code.static_id} {
+            if self.at_pos($i).code && %outers{self.at_pos($i).code.static_id}.defined {
                 @outers.push: $i;
                 return @outers if self.at_pos($i).is-routine;
             }

src/core/Exception.pm

@@ -10,9 +10,12 @@ my class Exception {
     }
 
     multi method gist(Exception:D:) {
-        my $str = try self.?message ~ "\n" ~ $.backtrace;
-        $! ?? "Error while creating error string: $!"
-           !! $str;
+        my $str = try self.?message;
+        return "Error while creating error string: $!" if $!;
+        $str ~= "\n";
+        try $str ~= self.backtrace;
+        return "$str\nError while creating backtrace: $!.message()\n$!.backtrace.full();" if $!;
+        return $str;
     }
 
     method throw() is hidden_from_backtrace {