DataFlash: expose the number of lost log messages

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libraries/DataFlash/DataFlash.cpp

@@ -203,6 +203,14 @@ void DataFlash_Class::Log_Write_Mission_Cmd(const AP_Mission &mission,
     FOR_EACH_BACKEND(Log_Write_Mission_Cmd(mission, cmd));
 }
 
+uint32_t DataFlash_Class::num_dropped() const
+{
+    if (_next_backend == 0) {
+        return 0;
+    }
+    return backends[0]->num_dropped();
+}
+
 
 // end functions pass straight through to backend
 

libraries/DataFlash/DataFlash.h

@@ -159,6 +159,9 @@ public:
 
     void periodic_tasks(); // may want to split this into GCS/non-GCS duties
 
+    // number of blocks that have been dropped
+    uint32_t num_dropped(void) const;
+    
     vehicle_startup_message_Log_Writer _vehicle_messages;
 
     // parameter support

libraries/DataFlash/DataFlash_Backend.h

@@ -94,6 +94,10 @@ public:
                              const AP_Param::ParamToken &token,
                              enum ap_var_type type);
 
+    uint32_t num_dropped(void) const {
+        return _dropped;
+    }
+    
 protected:
     uint32_t dropped;
     uint8_t internal_errors; // uint8_t - wishful thinking?