ScopedServiceProvider should throw helpful exception when outside HttpContext

[jaredcnance]

This often leads to exceptions like:

System.NullReferenceException : Object reference not set to an instance of an object.
---- The following constructor parameters did not have matching fixture data: TestFixture`1 fixture
Stack Trace:

----- Inner Stack Trace #1 (System.NullReferenceException) -----
   at JsonApiDotNetCore.Services.RequestScopedServiceProvider..ctor(IHttpContextAccessor httpContextAccessor)

JsonApiDotNetCore/src/JsonApiDotNetCore/Services/ScopedServiceProvider.cs

Lines 20 to 26 in 13404ab

	public RequestScopedServiceProvider(IHttpContextAccessor httpContextAccessor)
	{
	_httpContextAccessor = httpContextAccessor;
	}
	/// <inheritdoc />
	public object GetService(Type serviceType) => _httpContextAccessor.HttpContext.RequestServices.GetService(serviceType);