error: Signature errors should be returned with APIErrorCode.

The reasoning is that we can reply back with wide range of
S3 error responses, which would provide more richer context
to S3 client.

Fixes #1267

signature-v4-postpolicyform.go

@@ -159,51 +159,51 @@ func parsePostPolicyFormV4(policy string) (PostPolicyForm, *probe.Error) {
 }
 
 // checkPostPolicy - apply policy conditions and validate input values.
-func checkPostPolicy(formValues map[string]string) *probe.Error {
+func checkPostPolicy(formValues map[string]string) APIErrorCode {
 	if formValues["X-Amz-Algorithm"] != signV4Algorithm {
-		return ErrUnsuppSignAlgo("Unsupported signature algorithm in policy form data.", formValues["X-Amz-Algorithm"]).Trace(formValues["X-Amz-Algorithm"])
+		return ErrSignatureVersionNotSupported
 	}
 	/// Decoding policy
 	policyBytes, e := base64.StdEncoding.DecodeString(formValues["Policy"])
 	if e != nil {
-		return probe.NewError(e)
+		return ErrMalformedPOSTRequest
 	}
 	postPolicyForm, err := parsePostPolicyFormV4(string(policyBytes))
 	if err != nil {
-		return err.Trace()
+		return ErrMalformedPOSTRequest
 	}
 	if !postPolicyForm.Expiration.After(time.Now().UTC()) {
-		return ErrPolicyAlreadyExpired("Policy has already expired, please generate a new one.")
+		return ErrPolicyAlreadyExpired
 	}
 	if postPolicyForm.Conditions.Policies["$bucket"].Operator == "eq" {
 		if formValues["Bucket"] != postPolicyForm.Conditions.Policies["$bucket"].Value {
-			return ErrMissingFields("Policy bucket is missing.", formValues["Bucket"])
+			return ErrMissingFields
 		}
 	}
 	if postPolicyForm.Conditions.Policies["$x-amz-date"].Operator == "eq" {
 		if formValues["X-Amz-Date"] != postPolicyForm.Conditions.Policies["$x-amz-date"].Value {
-			return ErrMissingFields("Policy date is missing.", formValues["X-Amz-Date"])
+			return ErrMissingFields
 		}
 	}
 	if postPolicyForm.Conditions.Policies["$Content-Type"].Operator == "starts-with" {
 		if !strings.HasPrefix(formValues["Content-Type"], postPolicyForm.Conditions.Policies["$Content-Type"].Value) {
-			return ErrMissingFields("Policy content-type is missing or invalid.", formValues["Content-Type"])
+			return ErrMissingFields
 		}
 	}
 	if postPolicyForm.Conditions.Policies["$Content-Type"].Operator == "eq" {
 		if formValues["Content-Type"] != postPolicyForm.Conditions.Policies["$Content-Type"].Value {
-			return ErrMissingFields("Policy content-Type is missing or invalid.", formValues["Content-Type"])
+			return ErrMissingFields
 		}
 	}
 	if postPolicyForm.Conditions.Policies["$key"].Operator == "starts-with" {
 		if !strings.HasPrefix(formValues["Key"], postPolicyForm.Conditions.Policies["$key"].Value) {
-			return ErrMissingFields("Policy key is missing.", formValues["Key"])
+			return ErrMissingFields
 		}
 	}
 	if postPolicyForm.Conditions.Policies["$key"].Operator == "eq" {
 		if formValues["Key"] != postPolicyForm.Conditions.Policies["$key"].Value {
-			return ErrMissingFields("Policy key is missing.", formValues["Key"])
+			return ErrMissingFields
 		}
 	}
-	return nil
+	return ErrNone
 }