Updated the affected command to check if the local database is found, and to return an error if not.

Also updated the help text to indicate that a database is required.

Duplicati/CommandLine/help.txt

@@ -269,7 +269,7 @@ Usage: test-filters <source-path> [<options>]
 
 Usage: affected <storage-URL> <remote-filename> [<remote-filenames>] [<options>]
   
-  Returns a report explaining what backup sets and files are affected by a remote file. You can use this option to see what source files are affected if one or more remote files are damaged or deleted.
+  Returns a report explaining what backup sets and files are affected by a remote file. You can use this option to see what source files are affected if one or more remote files are damaged or deleted. Notes that this command requires a local database to be present, if the database is not found automatically, you can set --dbpath to point to the database to use.
 
 # =============================================================================
 > duplicati.commandline.exe help vacuum

Duplicati/Library/Main/Operation/ListAffected.cs

@@ -18,7 +18,8 @@
 using System;
 using System.Linq;
 using System.Collections.Generic;
-
+using Duplicati.Library.Interface;
+
 namespace Duplicati.Library.Main.Operation
 {
     internal class ListAffected
@@ -34,6 +35,9 @@ namespace Duplicati.Library.Main.Operation
             
         public void Run(List<string> args, Action<Duplicati.Library.Interface.IListAffectedResults> callback = null)
         {
+            if (!System.IO.File.Exists(m_options.Dbpath))
+                throw new UserInformationException(string.Format("Database file does not exist: {0}", m_options.Dbpath));
+
             using(var db = new Database.LocalListAffectedDatabase(m_options.Dbpath))
             {
                 m_result.SetDatabase(db);