remove failed keys

Author: Ravi Nagarjun Akella

include/sisl/cache/simple_cache.hpp

@@ -63,6 +63,15 @@ class SimpleCache {
             }
             t_evicted_keys.clear();
         }
+
+        // erase failed keys
+        if (t_failed_keys.size() > 0) {
+            for (const auto& key : t_failed_keys) {
+                V out_val;
+                m_map.erase(key, out_val);
+            }
+            t_failed_keys.clear();
+        }
         return ret;
     }