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/go/pki/locate.go b/go/pki/locate.go
index 3b4ca29..e075f55 100644
--- a/go/pki/locate.go
+++ b/go/pki/locate.go
@@ -16,7 +16,7 @@
 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