[perl #126936] LTA Error on is cached trait without use experimental

[via RT]

# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #126936]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=126936 >


When `is cached` trait is used without declaring `use experimental :cached` the 
error message does not mention the fact that experimental is missing, but it 
should:

<ZoffixW> m: sub yay($n) is cached { say "woop"; $n+1; }; say yay(1); say 
yay(1);
<camelia> rakudo-moar a54024: OUTPUT«===SORRY!=== Error while compiling 
/tmp/aGuHkHSdB6␤Can't use unknown trait 'is cached' in a sub declaration.␤at 
/tmp/aGuHkHSdB6:1␤    expecting any of:␤        rw raw hidden-from-backtrace 
hidden-from-USAGE␤        pure default DEPRECATED i…»

Even though it does mention it for macros:
<ZoffixW> m: macro x ($text){}
<camelia> rakudo-moar a54024: OUTPUT«===SORRY!=== Error while compiling 
/tmp/C1ZA2QCies␤Use of macros is experimental; please 'use experimental 
:macros'␤at /tmp/C1ZA2QCies:1␤------> macro⏏ x ($text){}␤»

Original discussion here: http://irclog.perlgeek.de/perl6/2015-12-16#i_11729206