Throw typed Exception for 'is rw' on optnl param

Added a new X::Trait::Invalid in Exception.pm that is then thrown in
BOOTSTRAP.nqp.

The new Exception might be generic enough to be used in other cases.

src/Perl6/Metamodel/BOOTSTRAP.nqp

@@ -1541,7 +1541,13 @@ BEGIN {
             }
             my int $flags := nqp::getattr_i($dcself, Parameter, '$!flags');
             if $flags +& $SIG_ELEM_IS_OPTIONAL {
-                nqp::die("Cannot use 'is rw' on optional parameter '$varname'");
+                my %ex := nqp::gethllsym('perl6', 'P6EX');
+                if nqp::isnull(%ex) || !nqp::existskey(%ex, 'X::Trait::Invalid') {
+                    nqp::die("Cannot use 'is rw' on optional parameter '$varname'");
+                }
+                else {
+                    nqp::atkey(%ex, 'X::Trait::Invalid')('is', 'rw', 'optional parameter', $varname);
+                }
             }
             my $cd := nqp::getattr($dcself, Parameter, '$!container_descriptor');
             if nqp::defined($cd) { $cd.set_rw(1) }

src/core/Exception.pm

@@ -781,6 +781,16 @@ my class X::Worry::P5::LeadingZero is X::Worry::P5 {
     }
 }
 
+my class X::Trait::Invalid is Exception {
+    has $.type;       # is, will, of etc.
+    has $.subtype;    # wrong subtype being tried
+    has $.declaring;  # variable, sub, parameter, etc.
+    has $.name;       # '$foo', '@bar', etc.
+    method message () {
+        "Cannot use '$.type $.subtype' on $.declaring '$.name'."
+    }
+}
+
 my class X::Trait::Unknown is Exception {
     has $.type;       # is, will, of etc.
     has $.subtype;    # wrong subtype being tried
@@ -2497,6 +2507,10 @@ nqp::bindcurhllsym('P6EX', nqp::hash(
       X::PhaserExceptions.new(exceptions =>
         @exceptions.map(-> Mu \e { EXCEPTION(e) })).throw;
   },
+  'X::Trait::Invalid',
+  sub ($type, $subtype, $declaring, $name) {
+      X::Trait::Invalid.new(:$type, :$subtype, :$declaring, :$name).throw;
+  },
 ));
 
 my class X::HyperWhatever::Multiple is Exception {