go/pki: fix error return

DeveloperCredentialsLocation used to glog.Exitf instead of returning an
error, and a consumer (prodaccess) used to not check the return code.
Bad refactor?

Change-Id: I6c2d05966ba6b3eb300c24a51584ccf5e324cd49
diff --git a/cluster/prodaccess/hspki.go b/cluster/prodaccess/hspki.go
index 2fcfaf0..04876fb 100644
--- a/cluster/prodaccess/hspki.go
+++ b/cluster/prodaccess/hspki.go
@@ -12,6 +12,9 @@
 
 func useHSPKIKeys(keys *pb.HSPKIKeys) {
 	path, err := pki.DeveloperCredentialsLocation()
+	if err != nil {
+		glog.Exitf("Could not get location of HSPKI creds: %v", err)
+	}
 	err = os.MkdirAll(path, 0700)
 	if err != nil {
 		glog.Exitf("mkdir %q: %v", path, err)