example: indicate when a sentence is not parsed or is not valid

Indicate when a sentence is not parsed or is not valid. Before this
change, it was not clear from the example run whether each sentence is
parsed properly or not.

example.c

@@ -34,6 +34,9 @@ int main(void)
                             minmea_tocoord(&frame.longitude),
                             minmea_tofloat(&frame.speed));
                 }
+                else {
+                    printf("$xxRMC sentence is not parsed\n");
+                }
             } break;
 
             case MINMEA_SENTENCE_GGA: {
@@ -41,6 +44,9 @@ int main(void)
                 if (minmea_parse_gga(&frame, line)) {
                     printf("$xxGGA: fix quality: %d\n", frame.fix_quality);
                 }
+                else {
+                    printf("$xxGGA sentence is not parsed\n");
+                }
             } break;
 
             case MINMEA_SENTENCE_GST: {
@@ -60,6 +66,9 @@ int main(void)
                             minmea_tofloat(&frame.longitude_error_deviation),
                             minmea_tofloat(&frame.altitude_error_deviation));
                 }
+                else {
+                    printf("$xxGST sentence is not parsed\n");
+                }
             } break;
 
             case MINMEA_SENTENCE_GSV: {
@@ -74,9 +83,17 @@ int main(void)
                             frame.sats[i].azimuth,
                             frame.sats[i].snr);
                 }
+                else {
+                    printf("$xxGSV sentence is not parsed\n");
+                }
+            } break;
+
+            case MINMEA_INVALID: {
+                printf("$xxxxx sentence is not valid\n");
             } break;
 
             default: {
+                printf("$xxxxx sentence is not parsed\n");
             } break;
         }
     }