fix: update expected error message when checking requester pays status

The error message recieved when querying for requester pays status changed to include an 'a' which caused strict matching to fail.
This fixes and standardizes the error message to use the same string for both the code and the tests.

Refs: googleapis#824

google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/CloudStorageFileSystemProvider.java

@@ -87,6 +87,8 @@
 @ThreadSafe
 public final class CloudStorageFileSystemProvider extends FileSystemProvider {
 
+  public static final String BUCKET_IS_REQUESTER_PAYS_ERROR_MESSAGE =
+      "Bucket is a requester pays bucket but no user project provided";
   private Storage storage;
   private final StorageOptions storageOptions;
   // if non-null, we pay via this project.
@@ -967,7 +969,7 @@ public boolean requesterPays(String bucketName) {
       Boolean isRP = storage.get(bucketName).requesterPays();
       return isRP != null && isRP.booleanValue();
     } catch (StorageException ex) {
-      if (ex.getCode() == 400 && ex.getMessage().contains("Bucket is requester pays")) {
+      if (ex.getCode() == 400 && ex.getMessage().contains(BUCKET_IS_REQUESTER_PAYS_ERROR_MESSAGE)) {
         return true;
       }
       throw ex;

google-cloud-nio/src/test/java/com/google/cloud/storage/contrib/nio/it/ITGcsNio.java

@@ -288,7 +288,7 @@ private void innerTestCantCopyWithoutUserProject(
       Assert.assertTrue(
           description,
           hex.getMessage()
-              .contains("Bucket is requester pays bucket but no user project provided"));
+              .contains(CloudStorageFileSystemProvider.BUCKET_IS_REQUESTER_PAYS_ERROR_MESSAGE));
     } catch (StorageException ex) {
       assertIsRequesterPaysException(description, ex);
     }
@@ -342,14 +342,16 @@ private void assertIsRequesterPaysException(String message, StorageException ex)
     Assert.assertEquals(message, ex.getCode(), 400);
     Assert.assertTrue(
         message,
-        ex.getMessage().contains("Bucket is requester pays bucket but no user project provided"));
+        ex.getMessage()
+            .contains(CloudStorageFileSystemProvider.BUCKET_IS_REQUESTER_PAYS_ERROR_MESSAGE));
   }
 
   private void assertIsRequesterPaysException(String message, IOException ioex) {
     Assert.assertTrue(message, ioex.getMessage().startsWith("400"));
     Assert.assertTrue(
         message,
-        ioex.getMessage().contains("Bucket is requester pays bucket but no user project provided"));
+        ioex.getMessage()
+            .contains(CloudStorageFileSystemProvider.BUCKET_IS_REQUESTER_PAYS_ERROR_MESSAGE));
   }
   // End of tests related to the "requester pays" feature