Recover from buggy HTML

On my sid system, check-mirrors.rb fails on one mirror whose web page
has invalid HTML:

    $ ./check-mirrors.rb --channel ${DIST:?} --allow-multiple --debug --fast tails-amd64-${VERSION:?} --url-prefix https://ftp.nluug.nl/os/Linux/distr/tails/tails/
    version:  tails-amd64-4.15
    fetch:    https://tails.boum.org/inc/trace
    trace:    1611584352

    mirror:   https://ftp.nluug.nl/os/Linux/distr/tails/tails/
    fetch:    https://ftp.nluug.nl/os/Linux/distr/tails/tails//project/trace
    trace:    1611584352
    fetch:    https://ftp.nluug.nl/os/Linux/distr/tails/tails//stable/
    Traceback (most recent call last):
    	7: from ./check-mirrors.rb:447:in `<main>'
    	6: from ./check-mirrors.rb:447:in `each'
    	5: from ./check-mirrors.rb:465:in `block in <main>'
    	4: from ./check-mirrors.rb:200:in `check_versions'
    	3: from ./check-mirrors.rb:187:in `scan_for_links'
    	2: from /usr/lib/ruby/vendor_ruby/nokogiri/html.rb:16:in `HTML'
    	1: from /usr/lib/ruby/vendor_ruby/nokogiri/html/document.rb:215:in `parse'
    /usr/lib/ruby/vendor_ruby/nokogiri/html/document.rb:215:in `read_memory': Parser without recover option encountered error or warning: 30:8: ERROR: Opening and ending tag mismatch: font and b (Nokogiri::XML::SyntaxError)
    zsh: exit 1     ./check-mirrors.rb --channel ${DIST:?} --allow-multiple --debug --fast

To workaround this, enable the Nokogiri RECOVER parser option,
whose documentation reads "Recover from errors":
https://nokogiri.org/rdoc/Nokogiri/XML/ParseOptions.html