chore(shorebird_cli): tweak public+private key error message

packages/shorebird_cli/lib/src/extensions/arg_results.dart

@@ -70,7 +70,7 @@ extension CodeSign on ArgResults {
       assertAbsentOrValidPublicKey();
       assertAbsentOrValidPrivateKey();
     } else {
-      logger.err('Both public and private keys must be provided or absent.');
+      logger.err('Both public and private keys must be provided.');
       exit(ExitCode.usage.code);
     }
   }

packages/shorebird_cli/test/src/commands/patch/patch_command_test.dart

@@ -354,7 +354,7 @@ void main() {
               );
               verify(
                 () => logger.err(
-                  'Both public and private keys must be provided or absent.',
+                  'Both public and private keys must be provided.',
                 ),
               ).called(1);
             });
@@ -380,7 +380,7 @@ void main() {
               );
               verify(
                 () => logger.err(
-                  'Both public and private keys must be provided or absent.',
+                  'Both public and private keys must be provided.',
                 ),
               ).called(1);
             });