net/http: return error from Serve if http2.ConfigureServer returns an error
In https://golang.org/cl/15860 http2.ConfigureServer was changed to
return an error if explicit CipherSuites are listed and they're not
compliant with the HTTP/2 spec.
This is the net/http side of the change, to look at the return value
from ConfigureServer and propagate it in Server.Serve.
h2_bundle.go will be updated in a future CL. There are too many other
http2 changes pending to be worth updating it now. Instead,
h2_bundle.go is minimally updated by hand in this CL so at least the
net/http change will compile.