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/policy.go

@@ -18,7 +18,6 @@ package policy
 
 import (
 	"encoding/json"
-	"fmt"
 	"io"
 )
 
@@ -78,7 +77,7 @@ func (policy Policy) IsEmpty() bool {
 // isValid - checks if Policy is valid or not.
 func (policy Policy) isValid() error {
 	if policy.Version != DefaultVersion && policy.Version != "" {
-		return fmt.Errorf("invalid version '%v'", policy.Version)
+		return Errorf("invalid version '%v'", policy.Version)
 	}
 
 	for _, statement := range policy.Statements {
@@ -108,7 +107,7 @@ func (policy Policy) isValid() error {
 				continue
 			}
 
-			return fmt.Errorf("duplicate principal %v, actions %v, resouces %v found in statements %v, %v",
+			return Errorf("duplicate principal %v, actions %v, resouces %v found in statements %v, %v",
 				principals, actions, resources, policy.Statements[i], statement)
 		}
 	}