Ensure error messages don't leak private key

Since #8766, invalid base64 is rendered in errors, but we don't actually
want to show this in the case of an invalid private keys.

Co-Authored-By: Eelco Dolstra <edolstra@gmail.com>

src/libutil/util.hh

@@ -172,9 +172,13 @@ constexpr char treeNull[] = "    ";
 
 
 /**
- * Base64 encoding/decoding.
+ * Encode arbitrary bytes as Base64.
  */
 std::string base64Encode(std::string_view s);
+
+/**
+ * Decode arbitrary bytes to Base64.
+ */
 std::string base64Decode(std::string_view s);