Parser and error-handling changes.

apps/modes_rx

@@ -227,7 +227,7 @@ if __name__ == '__main__':
           for out in outputs:
             try:
               out(msg.to_string())
-            except ADSBError:
+            except air_modes.ADSBError:
               pass
 
       elif runner.done:

python/modes_parse.py

@@ -75,9 +75,14 @@ class data_field:
   def get_bits(self, *args):
     startbit = args[0]
     num = args[1]
+    bits = 0
+    try:
       bits = (self.data \
           >> (self.get_numbits() - startbit - num + self.offset)) \
            & ((1 << num) - 1)
+    except ValueError:
+      #pass
+      print "Got short packet but expected long retrieving bits (%i, %i) with type %i" % (startbit, num, self.get_type())
     return bits
 
 class bds09_reply(data_field):

python/modes_print.py

@@ -217,7 +217,7 @@ class modes_output_print(modes_parse.modes_parse):
       retstr = "No handler for BDS1 == %i from %x" % (bds1, ecc)
 
     if(msgtype == 20):
-      retstr += " at %ift" % altitude
+      retstr += " at %ift" % alt
     else:
       retstr += " ident %x" % ident