Add type details to FilePathMarshaler error

To help diagnose #241...

Example error message if I replace a `Utf8Marshaler`:
```
	System.Runtime.InteropServices.MarshalDirectiveException : FilePathMarshaler must be used on a FilePath. Expected 'LibGit2Sharp.Core.FilePath' from 'C:\Dev\GitHub\libgit2sharp\LibGit2Sharp.Tests\bin\Debug\LibGit2Sharp.dll'; received 'System.String' from 'C:\Windows\Microsoft.NET\Framework\v2.0.50727\mscorlib.dll'.
	Core\FilePathMarshaler.cs(90,0): at LibGit2Sharp.Core.FilePathMarshaler.MarshalManagedToNative(Object managedObj)
```