improve the robustness of ndncert library

Change-Id: Iaabc4d8f28ca27a7e7f501ebd122c5231ceb3ac0
diff --git a/src/ca-config.hpp b/src/ca-config.hpp
index e20b8b4..0186067 100644
--- a/src/ca-config.hpp
+++ b/src/ca-config.hpp
@@ -65,6 +65,12 @@
   };
 
 public:
+  /**
+   * @throw CaConfig::Error when config file does not exist
+   * @throw CaConfig::Error when the JSON text in the file cannot be parsed correctly
+   * @throw CaConfig::Error when the ca-prefix attribute in JSON text is empty
+   * @throw CaConfig::Error when the challenge is not specified or is not supported
+   */
   void
   load(const std::string& fileName);