'exportConfig' requires an explicit filename and gives unhelpful errors

[Evmaus-MS]

If you run BinSkim's 'exportConfig' command with a folder as the output location, you get an unhelpful exception.

For example, if you run it with the desktop folder:
.\BinSkim.exe exportConfig "C:\Users{username}\Desktop"
System.IO.DirectoryNotFoundException: Could not find a part of the path 'C:\Users{username}\Desktop'.
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
at System.IO.StreamWriter.CreateFile(String path, Boolean append, Boolean checkHost)
at System.IO.StreamWriter..ctor(String path, Boolean append, Encoding encoding, Int32 bufferSize, Boolean checkHost)
at System.IO.File.InternalWriteAllText(String path, String contents, Encoding encoding, Boolean checkHost)
at Microsoft.CodeAnalysis.Sarif.Driver.ExportConfigurationCommandBase.Run(ExportConfigurationOptions exportOptions)

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The workaround is to explicitly specify a full filename (e.x. "binskim.exe exportconfig C:\users{username}\Desktop\binskimconfig.json"). However, the current behavior isn't particularly user friendly.

Expected behavior: BinSkim should either chose a default filename to output to if given a folder (e.x. 'binskimconfig.json' or 'binskimconfig.xml'), or give a more helpful error message (e.x. "Error: Please specify a full file name, not a folder"), rather than throwing an exception.