docs: better file not found example, remove file not found interface

Author: spacez320

README.md

@@ -132,8 +132,12 @@ You can handle the specific case where no config file is found like this:
 
 ```go
 if err := viper.ReadInConfig(); err != nil {
-	if _, ok := err.(viper.ConfigFileNotFoundError); ok {
-		// Config file not found; ignore error if desired
+	if errors.As(err, &viper.FileNotFoundError{}) {
+		// Indicates an explicitly set config file is not found, such as with
+		// using `viper.SetConfigFile`
+	} else if errors.As(err, &viper.FileNotFoundFromSearchError{}) {
+		// Indicates that no config file was found in any search path, such as
+		// when using `viper.AddConfigPath`
 	} else {
 		// Config file was found but another error was produced
 	}

errors.go

@@ -6,16 +6,6 @@ import (
 
 /* File look-up errors */
 
-// FileLookupError is returned when Viper cannot resolve a configuration file.
-//
-// This is meant to be a common interface for all file look-up errors, occurring either because a
-// file does not exist or because it cannot find any file matching finder criteria.
-type FileLookupError interface {
-	error
-
-	fileLookup()
-}
-
 // ConfigFileNotFoundError denotes failing to find a configuration file from a search.
 //
 // Deprecated: This is wrapped by FileNotFoundFromSearchError, which should be used instead.