[Bug] - REST PutBucketPolicy MalformedPolicy wording with no brackets provided

[lrm25]

Describe the bug
If a REST PutBucketPolicy command with a policy missing enclosing brackets is sent, an error similar to the following is returned:

<?xml version="1.0" encoding="UTF-8"?>
   <Error><Code>MalformedPolicy</Code><Message>invalid character &#39;d&#39; looking for beginning of value</Message><Resource></Resource><RequestId></RequestId><HostId></HostId></Error>

Or if sending something that can be parsed into a valid go type, such as false:

<?xml version="1.0" encoding="UTF-8"?>
   <Error><Code>MalformedPolicy</Code><Message>json: cannot unmarshal bool into Go value of type auth.BucketPolicy</Message><Resource></Resource><RequestId></RequestId><HostId></HostId></Error>

Though this shouldn't be too hard to figure out for someone who understands how policies should look, it's a bit confusing. Direct-to-s3 returns a more descriptive error:

<?xml version="1.0" encoding="UTF-8"?>
   <Error><Code>MalformedPolicy</Code><Message>Policies must be valid JSON and the first byte must be '{'</Message><RequestId>4382A7S00G3WWJZQ</RequestId><HostId>foAWoxHONj6IxWw1I8h3+LfRuSRGfSIQo6cCuikazzy/MzNzygo9oP8xRpDOjrYNnacABlsArlU=</HostId></Error>

The error message can be changed to this to avoid confusion.

To Reproduce

1. Start versitygw, e.g.: ./versitygw --access=AKIA**** --secret=******** --region=us-east-1 --cert /Users/lukemccrone/devel/versitygw/cert.pem --key /Users/lukemccrone/devel/versitygw/versitygw.pem posix --versioning-dir /tmp/versioning /tmp/gw
2. Create bucket, e.g.: aws --no-verify-ssl s3api create-bucket --bucket versity-gwtest-bucket-one-1
3. Attempt to run the script at https://github.com/versity/versitygw/blob/test/rest_put_bucket_policy/tests/rest_scripts/put_bucket_policy.sh with invalid policy data. The output should look like: curl -ks -w %{http_code} -X PUT https://localhost:7070/versity-gwtest-bucket-one-1?policy= -H Authorization: AWS4-HMAC-SHA256 Credential=********/20241122/us-east-1/s3/aws4_request,SignedHeaders=host;x-amz-content-sha256;x-amz-date,Signature=80f3ef779959b27958511885e19634a0cdd9c555d0656313145becf07d6aa5ef -H x-amz-content-sha256: fcbcf165908dd18a9e49f7ff27810176db8e9f63b4352213741664245224f8aa -H x-amz-date: 20241122T165320Z -d false -o /Users/lukemccrone/devel/versitygw/versity-gwtest-files/result.txt

Expected behavior
The policy error should be clearer.

Server Version
Version : v1.0.8
Build : bacd85c
BuildTime: 2024-11-21_05:29:52PM
Darwin Lukes-MacBook-Pro.local 23.4.0 Darwin Kernel Version 23.4.0: Fri Mar 15 00:12:25 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T6030 arm64