Unintuitive exception thrown for Excel 5.0/95 format worksheets

```
Steps to Reproduce:
1. Save an Excel worksheet using the format Microsoft Excel 5.0/95
2. Provide the path to that worksheet to DataSetHelper.CreateDataSet()
3. An ArgumentNullException is thrown from System.IO.MemoryStream.

There isn't much reason for excellibrary to support such an old format, but a
better error message might be useful. Is it possible to determine whether the
file is in the correct format? If so, perhaps the code could throw some more
specific Exception?

I'm using code checked out from the current trunk (r53).

Full StackTrace:
at System.IO.MemoryStream..ctor(Byte[] buffer, Boolean writable)
at System.IO.MemoryStream..ctor(Byte[] buffer)
at ExcelLibrary.SpreadSheet.Workbook.Load(Stream stream)
at ExcelLibrary.SpreadSheet.Workbook.Load(String file)
at ExcelLibrary.DataSetHelper.CreateDataSet(String filePath)
at TestExcelImport.Program.Main(String[] args) in c:\users\theuser\documents\visual studio 2010\Projects\TestExcelImport\TestExcelImport\Program.cs:line 12
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
```

Original issue reported on code.google.com by `etkl...@gmail.com` on 2 Jun 2011 at 6:28

Reproduce the failure with an Excel 5.0/95 worksheet, then trace the stack from DataSetHelper.CreateDataSet through Workbook.Load and the MemoryStream constructor. Inspect how the workbook format is read and define a clear failure for this input; done means it no longer exposes the current ArgumentNullException without an understandable explanation.

Written by the indexing model from the issue text.