curve25519: include potential fips140=only error in panic message

Updates golang/go#75061

Change-Id: I6a6a696474122a12c12696d8a2efec902572327d
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/696996
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: FiloSottile

curve25519/curve25519.go

@@ -36,7 +36,7 @@ func ScalarBaseMult(dst, scalar *[32]byte) {
 	curve := ecdh.X25519()
 	priv, err := curve.NewPrivateKey(scalar[:])
 	if err != nil {
-		panic("curve25519: internal error: scalarBaseMult was not 32 bytes")
+		panic("curve25519: " + err.Error())
 	}
 	copy(dst[:], priv.PublicKey().Bytes())
 }