Add more context aware error for policy parsing errors (#8726)

In existing functionality we simply return a generic
error such as "MalformedPolicy" which indicates just
a generic string "invalid resource" which is not very
meaningful when there might be multiple types of errors
during policy parsing. This PR ensures that we send
these errors back to client to indicate the actual
error, brings in two concrete types such as

 - iampolicy.Error
 - policy.Error

Refer #8202

pkg/policy/resource.go

@@ -18,7 +18,6 @@ package policy
 
 import (
 	"encoding/json"
-	"fmt"
 	"strings"
 
 	"github.com/minio/minio/pkg/policy/condition"
@@ -63,7 +62,7 @@ func (r Resource) Match(resource string, conditionValues map[string][]string) bo
 // MarshalJSON - encodes Resource to JSON data.
 func (r Resource) MarshalJSON() ([]byte, error) {
 	if !r.IsValid() {
-		return nil, fmt.Errorf("invalid resource %v", r)
+		return nil, Errorf("invalid resource %v", r)
 	}
 
 	return json.Marshal(r.String())
@@ -93,11 +92,11 @@ func (r *Resource) UnmarshalJSON(data []byte) error {
 // Validate - validates Resource is for given bucket or not.
 func (r Resource) Validate(bucketName string) error {
 	if !r.IsValid() {
-		return fmt.Errorf("invalid resource")
+		return Errorf("invalid resource")
 	}
 
 	if !wildcard.Match(r.BucketName, bucketName) {
-		return fmt.Errorf("bucket name does not match")
+		return Errorf("bucket name does not match")
 	}
 
 	return nil
@@ -106,14 +105,14 @@ func (r Resource) Validate(bucketName string) error {
 // parseResource - parses string to Resource.
 func parseResource(s string) (Resource, error) {
 	if !strings.HasPrefix(s, ResourceARNPrefix) {
-		return Resource{}, fmt.Errorf("invalid resource '%v'", s)
+		return Resource{}, Errorf("invalid resource '%v'", s)
 	}
 
 	pattern := strings.TrimPrefix(s, ResourceARNPrefix)
 	tokens := strings.SplitN(pattern, "/", 2)
 	bucketName := tokens[0]
 	if bucketName == "" {
-		return Resource{}, fmt.Errorf("invalid resource format '%v'", s)
+		return Resource{}, Errorf("invalid resource format '%v'", s)
 	}
 
 	return Resource{