make --allow-insecure-repositories message an error

Not using this option, but using unsigned (and co) repositories will
cause these repositories to be ignored and data acquiring from them
fails, so this is very well in the realms of an error and helps in
making 'apt-get update' fail with a non-zero error code as well.

apt-pkg/acquire-item.cc

@@ -1782,7 +1782,7 @@ void pkgAcqMetaSig::Failed(string Message,pkgAcquire::MethodConfig *Cnf)/*{{{*/
    } 
    else 
    {
-      _error->Warning("Use --allow-insecure-repositories to force the update");
+      _error->Error("Use --allow-insecure-repositories to force the update");
    }
 
    Item::Failed(Message,Cnf);
@@ -2161,7 +2161,7 @@ void pkgAcqMetaIndex::Failed(string Message,
       QueueIndexes(false);
    } else {
       // warn if the repository is unsinged
-      _error->Warning("Use --allow-insecure-repositories to force the update");
+      _error->Error("Use --allow-insecure-repositories to force the update");
       TransactionManager->AbortTransaction();
       Status = StatError;
       return;
@@ -2298,7 +2298,7 @@ void pkgAcqMetaClearSig::Failed(string Message,pkgAcquire::MethodConfig *Cnf) /*
          QueueIndexes(false);
       } else {
          // warn if the repository is unsigned
-         _error->Warning("Use --allow-insecure-repositories to force the update");
+         _error->Error("Use --allow-insecure-repositories to force the update");
          TransactionManager->AbortTransaction();
          Status = StatError;
       }