temp: Use more sane substitute value

When the ADC saturates, use a substitute value that doesn't kill the
following math...

Misc/Temp-Monitor/temperature_monitor.py

@@ -26,11 +26,12 @@ temperature_filtered = -1
 # Ignore the first 5 seconds
 last_report = time.ticks_ms() + 5000
 while True:
+    try:
         raw_value = cs1237.read()
         value = float(raw_value) / (2**24 - 1)
 
         if value > 0.9999:
-        value = 0
+            value = 20e3
         else:
             value = 10e3 * 1 / (1 / value - 1)
 
@@ -48,3 +49,5 @@ while True:
         if elapsed >= 200 or elapsed < 0:
             print(f"{temperature:.6f},{temperature_filtered:.6f}")
             last_report = now
+    except Exception as e:
+        print(f"Error {e}")