pldmd: Add details to InstanceIdDb::free error msg

and remove unnecessary `struct` from class variable;

Change-Id: Id7e76bd8f6123cc47d27cfc232a2a9186f2cad06
Signed-off-by: Rashmica Gupta <rashmica@linux.ibm.com>
diff --git a/pldmd/instance_id.hpp b/pldmd/instance_id.hpp
index 1f003fa..fae52c5 100644
--- a/pldmd/instance_id.hpp
+++ b/pldmd/instance_id.hpp
@@ -84,7 +84,9 @@
         int rc = pldm_instance_id_free(pldmInstanceIdDb, tid, instanceId);
         if (rc == -EINVAL)
         {
-            throw std::runtime_error("Invalid instance ID");
+            throw std::runtime_error(
+                "Instance ID " + std::to_string(instanceId) + " for TID " +
+                std::to_string(tid) + " was not previously allocated");
         }
         if (rc)
         {
@@ -93,7 +95,7 @@
     }
 
   private:
-    struct pldm_instance_db* pldmInstanceIdDb = nullptr;
+    pldm_instance_db* pldmInstanceIdDb = nullptr;
 };
 
 } // namespace pldm