Print meaningful ISP clock error message

Currently, only some JTAG3 compatible programmers are able to detect that the connected target's main clock speed actually is too low for for the programmer.
In this case, these will return 0xCC.

src/main.c

@@ -1242,8 +1242,11 @@ int main(int argc, char * argv [])
   init_ok = (rc = pgm->initialize(pgm, p)) >= 0;
   if (!init_ok) {
     pmsg_error("initialization failed, rc=%d\n", rc);
+    if(rc == -2)
+      imsg_error("The programmer ISP clock is too fast for the target\n");
+    else
       imsg_error("- double check the connections and try again\n");
-    imsg_error("- use -B to set lower ISP clock frequency, e.g. -B 200kHz\n");
+    imsg_error("- use -B to set lower ISP clock frequency, e.g. -B 125kHz\n");
     if (!ovsigck) {
       imsg_error("- use -F to override this check\n\n");
       exitrc = 1;

src/stk500v2.c

@@ -795,6 +795,11 @@ retry:
   // attempt to read the status back
   status = stk500v2_recv(pgm,buf,maxlen);
 
+  DEBUG("STK500V2: stk500v2_command() received content: [ ");
+  for (size_t i=0; i<len; i++)
+    DEBUG("0x%02x ",buf[i]);
+  DEBUG("], length %d\n", (int) len);
+
   // if we got a successful readback, return
   if (status > 0) {
     DEBUG(" = %d\n",status);
@@ -858,6 +863,9 @@ retry:
             return status;
         } else if (buf[1] == STATUS_CMD_FAILED) {
             pmsg_error("command failed\n");
+        } else if (buf[1] == STATUS_CLOCK_ERROR) {
+            pmsg_error("target clock speed error\n");
+            return -2;
         } else if (buf[1] == STATUS_CMD_UNKNOWN) {
             pmsg_error("unknown command\n");
         } else {