change RequiredFlagsErr to RequiredFlagErr, update to struct for custom message

flag.go

@@ -85,10 +85,12 @@ type RequiredFlag interface {
 
 // RequiredFlagsErr is an interface that allows users to redefine errors on required flags
 // it allows flags with user-defined errors to be backwards compatible with the Flag interface
-type RequiredFlagsErr interface {
+type RequiredFlagErr interface {
 	Flag
 
-	FlagsErrRequired() bool
+	IsCustom() bool
+	GetMessage() string
+	HasInterpolation() bool
 }
 
 // DocGenerationFlag is an interface that allows documentation generation for the flag