add error handling for addRequest

Change-Id: I46dfef76735f14abc1dc8d0a1d618a1963d4f8a0
diff --git a/src/ca-storage-detail/ca-memory.hpp b/src/ca-storage-detail/ca-memory.hpp
index eeb3397..754090e 100644
--- a/src/ca-storage-detail/ca-memory.hpp
+++ b/src/ca-storage-detail/ca-memory.hpp
@@ -33,10 +33,15 @@
   const static std::string STORAGE_TYPE;
 
 public:
-  // certificate request related
+  /**
+   * @throw if request cannot be fetched from underlying data storage
+   */
   CaState
   getRequest(const std::string& requestId) override;
 
+  /**
+   * @throw if there is an existing request with the same request ID
+   */
   void
   addRequest(const CaState& request) override;