Tell user if instance addition fails

[HebaruSan]

Problem

If a user tries to add a game instance for which they lack filesystem write privileges, it fails silently, and an exception appears in the terminal:

System.UnauthorizedAccessException: Access to the path '/ram/Fake-1.11.1/CKAN' is denied. ---> System.IO.IOException: Keine Berechtigung
--- End of inner exception stack trace ---
at System.IO.FileSystem.CreateDirectory (System.String fullPath) [0x00191] in <673b2a9c49de4736b9b01249f8d6d6b1>:0
at System.IO.Directory.CreateDirectory (System.String path) [0x0002c] in <673b2a9c49de4736b9b01249f8d6d6b1>:0
at ChinhDo.Transactions.FileManager.TxFileManager.CreateDirectory (System.String path) [0x0001b] in <bd256a5f4bd9434c90d5e776dca9de7b>:0
at CKAN.GameInstance.SetupCkanDirectories (System.Boolean scan) [0x0006e] in <bd256a5f4bd9434c90d5e776dca9de7b>:0
at CKAN.GameInstance..ctor (CKAN.Games.IGame game, System.String gameDir, System.String name, CKAN.IUser user, System.Boolean scan) [0x0007e] in <bd256a5f4bd9434c90d5e776dca9de7b>:0
at CKAN.GameInstanceManager.AddInstance (System.String path, System.String name, CKAN.IUser user) [0x00053] in <bd256a5f4bd9434c90d5e776dca9de7b>:0
at CKAN.ManageGameInstancesDialog.AddToCKANMenuItem_Click (System.Object sender, System.EventArgs e) [0x0005f] in <bd256a5f4bd9434c90d5e776dca9de7b>:0

Most users don't look at the terminal, so this is confusing.

Cause

We try to create the <KSP>/CKAN folder, which fails, and we don't catch the exception.

Changes

Now if an exception is thrown when we try to add an instance, we display the exception's Message in an error popup. This way the user should have some clue why it failed.

Also fixed two minor compiler warnings to do with TarInputStream wanting an encoding parameter and a catch block not using a parameter.

Fixes #3330.

[DasSkelett]

Solves it indeed, thanks a bunch!