Output reason for 400 when SNI host name != HTTP host name

We can annotate the default error output by using r->notes.  Do so for
the case of SNI host name != HTTP host name, since it's an error that
confuses people a lot.
Merge pull request #9 from fastly/tfheen/sni-http-host-header-mismatch

Output reason for 400 when SNI host name != HTTP host name