fix: Fixes the bucket policy json document validation errors for invalid jsons.

Fixes #965

Changes the returned error description to `Policies must be valid JSON and the first byte must be '{'` for invalid bucket policy json documents, which doesn't start with `{`.
The gateway returns `This policy contains invalid Json` error description, if the document starts with `{`, but still isn't valid json.

Implements the `policyErr` string type which implements the `error` interface, to handle the policy json document validation errors, by avoiding staticchecker warnings.

s3api/controllers/base_test.go

@@ -634,8 +634,7 @@ func TestS3ApiController_PutBucketActions(t *testing.T) {
 	</VersioningConfiguration>
 	`
 
-	policyBody := `
-	{
+	policyBody := `{
 		"Statement": [
 			{
 				"Effect": "Allow",