Ensure Magneto throws meaningful exception when context is not regist…

…ered

src/Magneto/Core/ServiceProviderExtensions.cs

@@ -1,9 +1,20 @@
-using System;
+using System;
 
 namespace Magneto.Core
 {
 	static class ServiceProviderExtensions
 	{
 		internal static T GetService<T>(this IServiceProvider serviceProvider) => (T)serviceProvider.GetService(typeof(T));
+
+		internal static T GetRequiredService<T>(this IServiceProvider serviceProvider)
+		{
+			var serviceType = typeof(T);
+			var service = serviceProvider.GetService(serviceType);
+
+			if (service == null)
+				throw new InvalidOperationException($"No service for type '{serviceType.FullName}' has been registered.");
+
+			return (T)service;
+		}
 	}
 }

src/Magneto/Magneto.cs

@@ -17,7 +17,7 @@ namespace Magneto
 	/// and commands to express the required context as being a composite of several items. For example:
 	/// </para>
 	/// <example>
-	/// <c>class MyQuery : ISyncQuery&lt;(DbContext, IFileProvider), string&gt; { ... }</c>
+	/// class MyQuery : ISyncQuery&lt;(DbContext, IFileProvider), string&gt; { ... }
 	/// </example>
 	/// </summary>
 	public class Magneto : IMagneto
@@ -56,7 +56,8 @@ public Magneto(IServiceProvider serviceProvider)
 		/// </summary>
 		/// <typeparam name="TContext">The type of context object required.</typeparam>
 		/// <returns>The result from calling <see cref="IServiceProvider.GetService"/>.</returns>
-		protected virtual TContext GetContext<TContext>() => ServiceProvider.GetService<TContext>();
+		/// <exception cref="InvalidOperationException">Thrown if the <see cref="ServiceProvider"/> couldn't provide it.</exception>
+		protected virtual TContext GetContext<TContext>() => ServiceProvider.GetRequiredService<TContext>();
 
 		/// <inheritdoc cref="ISyncQueryMagneto.Query{TContext,TResult}"/>
 		public virtual TResult Query<TContext, TResult>(ISyncQuery<TContext, TResult> query) =>

test/Magneto.Tests/MagnetoTests/MagnetoTests.cs

@@ -20,6 +20,16 @@ public override void Setup()
 			SUT = new MagnetoTest(_serviceProvider);
 		}
 
+		public class ForUnavailableType : GettingContext
+		{
+			Func<Attribute> _invocation;
+
+			void WhenContextIsResolved() => _invocation = SUT.Invoking(x => x.ResolveContext<Attribute>());
+
+			void ThenItThrowsAnExceptionStatingThatTheServiceIsNotRegistered() => _invocation.Should().Throw<InvalidOperationException>()
+				.Which.Message.Should().Be("No service for type 'System.Attribute' has been registered.");
+		}
+
 		public class ForNormalType : GettingContext
 		{
 			void WhenContextIsResolved()