Gracefully handle errors applying redlinks (wmf7)

An invalid wikilink like [[Foo&bar]], when it gets through to the
redlink handling, will error out because &bar is not a valid html
entity.  This first repair sets up more gracefull error handling
for failures in redlinking(just show the content without redlinks
applied).  Also partial fix for underlying problem of invalid
entities.

(Backported to 1.23wmf7)

Bug: 58455
Change-Id: Iaa36485325668af4bb7b4556e6dedf9c9d3bf346