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

cluster/prodaccess/hspki.go

@@ -12,6 +12,9 @@ import (
 
 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)

go/pki/locate.go

@@ -16,7 +16,7 @@ import (
 func DeveloperCredentialsLocation() (string, error) {
 	cfgDir, err := os.UserConfigDir()
 	if err != nil {
-		glog.Exitf("UserConfigDir: %w", err)
+		return "", fmt.Errorf("UserConfigDir: %w", err)
 	}
 
 	return fmt.Sprintf("%s/hspki", cfgDir), nil