Rearrange validation so that the user only ever sees errors

*after* providing input.

In particular if the existing config does not validate then we
should just ensure that the problematic question is displayed
without a confusing error message beforehand; the error should
only appear if the newly entered response doesn't validate.

debian/dump1090-mutability.config

@@ -62,28 +62,32 @@ db_input_verify() {
   db_input $PRI $KEY; RESULT=$?
   db_go
   set -e
+  ASKED=0
   while :
   do
     db_get $KEY
     if $VERIFY $RET; then return 0; fi
     if [ $RESULT -ne 0 ]; then
       # db_input failed, and the existing value does not validate
-      if [ $RESULT = 30 ] && [ $PRI != high ]
+      if [ $RESULT = 30 ] && [ $ASKED = 0 ]
       then
-        # question wasn't displayed, but existing value is invalid
-        # bump priority and try again
-        PRI=high
+       # question was skipped, but existing value is invalid
+       # bump priority and try again (once)
+       PRI=high
+       ASKED=1
       else
        # give up, use the default value
        db_reset $KEY
        return 0
       fi
+    else 
+     # db_input was OK, but the value did not verify.
+     # show an error message
+     db_input high dump1090-mutability/invalid-$VERIFY || true
     fi
 
-    # db_input was OK, but the value did not verify.
-    # display an error and try again.   
+    # try again
     set +e
-    db_input high dump1090-mutability/invalid-$VERIFY
     db_fset $KEY seen false
     db_input high $KEY; RESULT=$?
     db_go