CLI doesn't declare a syntax error for invalid --overwrite parameter

The CLI has --no-overwrite and --overwrite-metadata parameters. It has no documented --overwrite parameter. This should be a syntax error.

Environment:
* Linux ip-172-31-39-147 4.9.58-18.55.amzn1.x86_64 #1 SMP Thu Nov 2 04:38:47 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
* aws-encryption-sdk-cli/1.0.2.6 aws-encryption-sdk/1.3.2

```
aws-crypto --encrypt --input myData \
--master-keys key=$cmkArn provider='aws-kms' \
--output myEncryptedMessage \
--encryption-context purpose=test \
--metadata-output ~/metadata \
--overwrite
2017-11-17 16:32:08,975 - MainThread - aws_encryption_sdk_cli - WARNING - Overwriting existing target file because no action was specified otherwise: myEncryptedMessage

(ENV) [ec2-user@ip-172-31-39-147 test]$ echo $?
0
```