Check if bucket exists before attempting to delete it

If you attempt to delete a bucket if it does not exist, the API returns a `404` for the not existent bucket, which is correct. So before the attempt to remove the bucket, there should be a check if it even exists.

This test-bug was introduced in commit 9772a373b9, which also disabled the error handling for all requests - which is the reason this bug was not discovered so far.

Minio.Functional.Tests/FunctionalTest.cs

@@ -364,7 +364,11 @@ public static class FunctionalTest
 
         try
         {
-            await minio.RemoveBucketAsync(rbArgs).ConfigureAwait(false);
+            if (await minio.BucketExistsAsync(beArgs).ConfigureAwait(false))
+            {
+                await minio.RemoveBucketAsync(rbArgs).ConfigureAwait(false);
+            }
+
             var found = await minio.BucketExistsAsync(beArgs).ConfigureAwait(false);
             Assert.IsFalse(found);
             await minio.MakeBucketAsync(mbArgs).ConfigureAwait(false);