Fix p6doc build errors

Build.pm

@@ -2,6 +2,7 @@
 # need this to install pod under .../share/perl6/doc
 # This fixes p6doc command line use.
 
+use v6;
 use File::Find;
 
 class Build {
@@ -11,7 +12,7 @@ class Build {
         mkdir($dest-pref) unless $dest-pref.d;
 
         my @files = find(dir => "$workdir/doc", type => 'file').list;
-
+        my $copied-all = True;
         for @files -> $file {
             next if $file.basename eq 'HomePage.pod6';
 
@@ -23,7 +24,11 @@ class Build {
             say "Copying {$rel-dest} to {$abs-dest}";
 
             copy($file, $abs-dest);
+            $copied-all = False unless $abs-dest.IO.e;
         }
+
+        # Zef considers the build to have passed only if a truthy value is returned.
+        $copied-all;
     }
     method isa($what) { # Only needed for older panda compatibility
         return True if $what.^name eq 'Panda::Builder';