Fixed yaml bug with leading spaces

The yaml decoder could not read yaml files that used spaces instead of tabs.

I also improved the error output for the yaml reader a little. 
Replace this with:
```csharp
var tabs = MiniYaml.FromString(yamlTabStyle, "yamlTabStyle").WriteToString();
var mixed = MiniYaml.FromString(yamlMixedStyle, "yamlMixedStyle").WriteToString();
Assert.AreEqual(tabs, mixed);
```

You can add

```csharp
Console.WriteLine(tabs);
Console.WriteLine(mixed);
```

in between to confirm.