Write the bad-whitespace pattern to the search register

Add a command that writes the bad-whitespace pattern to the '/'
register.  This way you can jump to an error by creating a mapping like
this:

    nnoremap <Leader>W :<C-U>SetSearchPatternToBadWhitespace<CR>n
Use the buffer specific pattern to erase whitespace

One assumes that only the highlighted whitespace will be removed by that
command.