aws s3 cp without recursive can produce a useless error message

Without `--recursive`:
```
aws s3 cp ./{DIRECTORY} s3://{BUCKET}/
upload failed: {DIRECTORY}/ to s3://{BUCKET}/ Parameter validation failed:
Invalid length for parameter Key, value: 0, valid range: 1-inf
```

With `--recursive`:
```
aws s3 cp --recursive ./{DIRECTORY} s3://{BUCKET}/
upload: ...
```

When searching for that error, the first result on google is about issue #2929 which as been closed for no apparent reason.

This comment says it all https://github.com/aws/aws-cli/issues/2929#issuecomment-393869946

Please improve that message...

Version info:
```
➜ workspace aws --version
aws-cli/1.15.50 Python/3.7.0 Darwin/17.7.0 botocore/1.10.49
```