improve robustness of internal error reporting

src/core/Backtrace.pm

@@ -121,7 +121,7 @@ my class Backtrace is List {
             return @frames.join;
             CATCH {
                 default {
-                    return "<Internal error while creating backtrace: $!.message().\n"
+                    return "<Internal error while creating backtrace: $_.message().\n"
                         ~ "Please report this as a bug (mail to rakudobug@perl.org)\n", 
                         ~ "and re-run with the --ll-exception command line option\n"
                         ~ "to get more information about your error>";