Fix JSONizing exceptions, spotted by Garland_g[m]++

When running with RAKUDO_EXCEPTIONS_HANDLER=JSON, any Exception that
contains another Exception as one of the attributes, would throw.  And
since we apparently don't have a CATCHer there, would give a very LTA
error message.  Fixed by moving the JSONizing of an Exception to
Rakudo::Internals::JSON, so it can recursively handle itself.  And
adapt the exception handling in Exception accordingly.

src/core/Exception.pm

@@ -2904,19 +2904,24 @@ my class X::CompUnit::UnsatisfiedDependency is Exception {
 }
 
 my class Exceptions::JSON {
+    sub jsonify($ex) {
+        Rakudo::Internals::JSON.to-json( $ex.^name => Hash.new(
+          (message => $ex.?message),
+          $ex.^attributes.grep(*.has_accessor).map: {
+              with .name.substr(2) -> $attr {
+                  $attr => (
+                    (.defined and not $_ ~~ Real|Positional|Associative)
+                      ?? .Str
+                      !! $_ ~~ Exception
+                        ?? jsonify($_)
+                        !! $_
+                  ) given $ex."$attr"()
+              }
+          }
+        ))
+    }
     method process($ex) {
-        $*ERR.print:
-          Rakudo::Internals::JSON.to-json( $ex.^name => Hash.new(
-            (message => $ex.?message),
-            $ex.^attributes.grep(*.has_accessor).map: {
-                with .name.substr(2) -> $attr {
-                    $attr => (
-                        (.defined and not $_ ~~ Real|Positional|Associative)
-                        ?? .Str !! $_
-                    ) given $ex."$attr"()
-                }
-            }
-          ));
+        $*ERR.print: Rakudo::Internals::JSON.to-json($ex);
         False  # done processing
     }
 }

src/core/Rakudo/Internals/JSON.pm

@@ -120,6 +120,20 @@ my class Rakudo::Internals::JSON {
                 ~ '}';
     }
 
+    multi sub to-json(Exception:D $ex, :$indent = 0; :$first = 0) {
+       to-json( $ex.^name => Hash.new(
+          (message => $ex.?message),
+          $ex.^attributes.grep(*.has_accessor).map: {
+              with .name.substr(2) -> $attr {
+                  $attr => (
+                    (.defined and not $_ ~~ Real|Positional|Associative)
+                      ?? .Str !! $_
+                  ) given $ex."$attr"()
+              }
+          }
+          ), :$indent, :$first )
+    }
+
     multi sub to-json(Mu:U $, :$indent = 0, :$first = 0) { 'null' }
     multi sub to-json(Mu:D $s, :$indent = 0, :$first = 0) {
         die "Can't serialize an object of type " ~ $s.WHAT.perl