Better message when ZipFile throws NotSupportedException

[HebaruSan]

Problem

See #3066 and #3123; if Mono users in a non-en_US locale don't install mono-complete, this exception can be thrown:

System.NotSupportedException: Encoding 852 data could not be found. Make sure you have correct international codeset assembly installed and enabled.
  at System.Text.Encoding.GetEncoding (System.Int32 codepage) [0x0024d] in <12b418a7818c4ca0893feeaaf67f1e7f>:0 
  at ICSharpCode.SharpZipLib.Zip.ZipConstants.ConvertToString (System.Byte[] data, System.Int32 count) [0x00017] in <29fa7683355b471a9cc202367184a830>:0 
  at ICSharpCode.SharpZipLib.Zip.ZipConstants.ConvertToStringExt (System.Int32 flags, System.Byte[] data, System.Int32 count) [0x00032] in <29fa7683355b471a9cc202367184a830>:0 
  at ICSharpCode.SharpZipLib.Zip.ZipFile.ReadEntries () [0x00312] in <29fa7683355b471a9cc202367184a830>:0 
  at ICSharpCode.SharpZipLib.Zip.ZipFile..ctor (System.String name) [0x0006b] in <29fa7683355b471a9cc202367184a830>:0 
  at CKAN.NetFileCache.ZipValid (System.String filename, System.String& invalidReason) [0x00003] in <29fa7683355b471a9cc202367184a830>:0 

Cause

SharpZipLib apparently uses some kind of locale-specific code in its ZIP validation logic, which does not like being called without the locale data installed.

Changes

Now we catch that exception on Mono and tell the user to install mono-complete. The mod installation will still fail, because we need to confirm the file is valid, and we can't do that if this is thrown.

[DasSkelett]

Inserted throw new NotSupportedException() manually since I didn't wan't to uninstall mono-complete, and there it works. Good enough for me.