AP_Logger: add error to PID message

libraries/AP_Logger/AP_Logger.h

@@ -285,6 +285,7 @@ public:
     struct PID_Info {
         float target;
         float actual;
+        float error;
         float P;
         float I;
         float D;

libraries/AP_Logger/LogFile.cpp

@@ -797,6 +797,7 @@ void AP_Logger::Write_PID(uint8_t msg_type, const PID_Info &info)
         time_us         : AP_HAL::micros64(),
         target          : info.target,
         actual          : info.actual,
+        error           : info.error,
         P               : info.P,
         I               : info.I,
         D               : info.D,

libraries/AP_Logger/LogStructure.h

@@ -726,6 +726,7 @@ struct PACKED log_PID {
     uint64_t time_us;
     float   target;
     float   actual;
+    float   error;
     float   P;
     float   I;
     float   D;
@@ -1224,10 +1225,10 @@ struct PACKED log_Arm_Disarm {
 #define MAG_UNITS "sGGGGGGGGG-s"
 #define MAG_MULTS "FCCCCCCCCC-F"
 
-#define PID_LABELS "TimeUS,Des,Act,P,I,D,FF"
-#define PID_FMT    "Qffffff"
-#define PID_UNITS  "s------"
-#define PID_MULTS  "F------"
+#define PID_LABELS "TimeUS,Des,Act,Err,P,I,D,FF"
+#define PID_FMT    "Qfffffff"
+#define PID_UNITS  "s-------"
+#define PID_MULTS  "F-------"
 
 #define QUAT_LABELS "TimeUS,Q1,Q2,Q3,Q4"
 #define QUAT_FMT    "Qffff"