[perl #125620] Exception subclasses without a message method aren't handled well by the top level exception handler

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# New Ticket Created by  Rob Hoelz 
# Please include the string:  [perl #125620]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=125620 >


For example:

    $ perl6 -e 'class CustomException is Exception {}; 
CustomException.new.throw'

This results in the following error output:

Unhandled exception: concatenate requires a concrete string, but got null
   at <unknown>:1  
(/home/rob/.mokudo/share/perl6/runtime/CORE.setting.moarvm:print_exception:4294967295)
 from src/gen/m-CORE.setting:15582  
(/home/rob/.mokudo/share/perl6/runtime/CORE.setting.moarvm:<anon>:43)
 from gen/moar/stage2/NQPHLL.nqp:1420  
(/home/rob/.mokudo/share/nqp/lib/NQPHLL.moarvm:command_eval:380)
 from src/Perl6/Compiler.nqp:59  
(/home/rob/.mokudo/share/nqp/lib/Perl6/Compiler.moarvm:command_eval:93)
 from gen/moar/stage2/NQPHLL.nqp:1360  
(/home/rob/.mokudo/share/nqp/lib/NQPHLL.moarvm:command_line:116)
 from src/gen/m-main.nqp:39  
(/home/rob/.mokudo/share/perl6/runtime/perl6.moarvm:MAIN:18)
 from src/gen/m-main.nqp:35  
(/home/rob/.mokudo/share/perl6/runtime/perl6.moarvm:<mainline>:197)
 from <unknown>:1  (/home/rob/.mokudo/share/perl6/runtime/perl6.moarvm:<main>:8)
 from <unknown>:1  
(/home/rob/.mokudo/share/perl6/runtime/perl6.moarvm:<entry>:9)

Adding a message method to CustomException causes the exception handler to 
print out more normal-looking output.