prevent xargs from quiting with 123

Some repositories have runs of file-like-objects which the word splitter
is unable to read. We want to keep processing readable files.

spelling-unknown-word-splitter.pl

@@ -19,7 +19,7 @@
   @ARGV = grep {-r || $_ eq '-'} @ARGV;
   unless (scalar @ARGV) {
     print STDERR "None of the provided files are readable\n";
-    exit 1;
+    exit 0;
   }
 }