html.c: die when write fails

If we fail to write HTML output once, there's no point carrying on so
just write a failure message once and die.  By using Git's die_errno
function we also let the user know in what way the write failed.

Signed-off-by: John Keeping <john@keeping.me.uk>
shared.c: use die_errno() where appropriate

This replaces some code that is re-implementing die_errno by just
calling the function.

Signed-off-by: John Keeping <john@keeping.me.uk>