Bug: synthesizer fails due to automatically added system hidden files, error does not contain a message

Hello,

After debugging my issue I've found, that function `validateFilesAndReturnPath()` from `export-backend-asset-handler` file fails due to this check:
```
const allFiles = fs.readdirSync(filePath);
if (allFiles?.length > 1) {
throw new Error();
}
```

I see 2 issues there:
1. Firstly, error doesn't have description. If you get it (like I did), then CDK synthesizer will just throw an error without any description, which is very user non friendly.
2. And the root cause - if there are automatically generated files, like for example `.DS_Store` file on Mac, then it will fail, because number of files found is greater than 1.

I believe this might be a common issue, since you need to unzip function package manually and then zip it again, if you want to change anything in Lambda function, which is a story for another GitHub issue. I'm not sure, but I believe this file is created after that operation.

Start in the export-backend-asset-handler file at validateFilesAndReturnPath(), especially the fs.readdirSync(filePath) check. Reproduce the failure with an automatically generated hidden file such as .DS_Store and inspect the resulting error. Done means valid Lambda package handling no longer fails solely because of that file, and failures include a useful message.

Written by the indexing model from the issue text.