test: RelocationTests.testCancellationCleansTempFiles may fail due to io errors while check temp files are deleted

Windows can throw NoSuchFileException when using File.walkFileTree and deleting files concurrently. This commit changes IO exceptions into assertion error so that assertBusy will wait for them as well.

src/test/java/org/elasticsearch/recovery/RelocationTests.java

@@ -516,7 +516,7 @@ public class RelocationTests extends ElasticsearchIntegrationTest {
                                 }
                             });
                         } catch (IOException e) {
-                            throw new ElasticsearchException("failed to walk tree", e);
+                            throw new AssertionError("failed to walk file tree starting at [" + shardLoc.toPath() + "]", e);
                         }
                     }
                 });