Check that NewWritableFile succeeded when copying over backup files (facebook#13734)

Summary:
I am seeing crashes during backups. The stack trace points back to `WritableFileWriter` creation inside `BackupEngineImpl::CopyOrCreateFile`. I believe the issue is that we are calling `writable_file_->GetRequiredBufferAlignment()` with a `null` `writable_file`.

https://github.com/facebook/rocksdb/blob/v10.2.1/utilities/backup/backup_engine.cc#L2396-L2397

https://github.com/facebook/rocksdb/blob/v10.2.1/file/writable_file_writer.h#L210

Here's how I think the flow is:

```cpp
  io_s = dst_env->GetFileSystem()->NewWritableFile(dst, dst_file_options,
                                                   &dst_file, nullptr);
// say there was some issue and dst_file is nullptr
// evaluates to false
if (io_s.ok() && !src.empty()) {
   // we don't go down this branch
    auto src_file_options = FileOptions(src_env_options);
    src_file_options.temperature = *src_temperature;
    io_s = src_env->GetFileSystem()->NewSequentialFile(src, src_file_options,
                                                       &src_file, nullptr);
  }
  // say this evaluates to true
  if (io_s.IsPathNotFound() && *src_temperature != Temperature::kUnknown) {
    // Retry without temperature hint in case the FileSystem is strict with
    // non-kUnknown temperature option
    io_s = src_env->GetFileSystem()->NewSequentialFile(
        src, FileOptions(src_env_options), &src_file, nullptr);
  }
// this is now from the NewSequentialFile call, not NewWritableFile
  if (!io_s.ok()) {
    return io_s;
  }
// dst_file is still nullptr
```

If the first `NewWritableFile` fails and `IsPathNotFound

Tests: existing unit tests

Pull Request resolved: facebook#13734

Reviewed By: pdillinger

Differential Revision: D77390694

Pulled By: archang19

fbshipit-source-id: 865a3a646079ae2349a3b6f25e53ae85df8e4985

Author: archang19

utilities/backup/backup_engine.cc

@@ -2372,7 +2372,11 @@ IOStatus BackupEngineImpl::CopyOrCreateFile(
 
   io_s = dst_env->GetFileSystem()->NewWritableFile(dst, dst_file_options,
                                                    &dst_file, nullptr);
-  if (io_s.ok() && !src.empty()) {
+  if (!io_s.ok()) {
+    return io_s;
+  }
+
+  if (!src.empty()) {
     auto src_file_options = FileOptions(src_env_options);
     src_file_options.temperature = *src_temperature;
     io_s = src_env->GetFileSystem()->NewSequentialFile(src, src_file_options,