crypto/tls: handle errors in generate_cert.go

I don't expect these to hit often, but we should still alert users if
we fail to write the correct data to the file, or fail to close it.

Change-Id: I33774e94108f7f18ed655ade8cca229b1993d4d2
Reviewed-on: https://go-review.googlesource.com/91456


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>