GetModelContentType: throw exception when no UmbracoContext exists

[ronaldbarendse]

This 'fixes' issue #222 and should be able to get merged with PR #223.

Instead of throwing an ArgumentNullException, this method now throws an InvalidOperationException indicating it requires an UmbracoContext to be present.

To make it all work, callers should ensure this using IUmbracoContextFactory.EnsureUmbracoContext() as described in #222 (comment).

If you only need the property alias, the new GetModelPropertyTypeAlias method from PR #223 (comment) doesn't require the UmbracoContext.

[ronaldbarendse]

After merging with #223, the following XML documentation should be added to the GetModelContentType method (above the ArgumentOutOfRangeException):

/// <exception cref="InvalidOperationException">This method requires an UmbracoContext, ensure this is created using IUmbracoContextFactory.EnsureUmbracoContext().</exception>