Update docs

docs/scenarios/assertions.md

@@ -6,7 +6,7 @@ failures will be reported out in the Exception message thrown by Alba on Scenari
 :::
 
 The Scenario assertions in Alba are completely extensible and you can happily add your own via extension methods - but
-please send anything that's generally useful as a pull request to Alba itself;-)
+please send anything that's generally useful as a pull request to Alba itself ;-)
 
 The first step is to write your own implementation of this interface:
 
@@ -15,7 +15,7 @@ The first step is to write your own implementation of this interface:
 ```cs
 public interface IScenarioAssertion
 {
-    void Assert(Scenario scenario, HttpContext context, ScenarioAssertionException ex);
+    void Assert(Scenario scenario, AssertionContext context);
 }
 ```
 <sup><a href='https://github.com/JasperFx/alba/blob/master/src/Alba/IScenarioAssertion.cs#L5-L10' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_iscenarioassertion' title='Start of snippet'>anchor</a></sup>
@@ -35,19 +35,21 @@ internal sealed class BodyContainsAssertion : IScenarioAssertion
         Text = text;
     }
 
-    public void Assert(Scenario scenario, HttpContext context, ScenarioAssertionException ex)
+    public void Assert(Scenario scenario, AssertionContext context)
     {
-        var body = ex.ReadBody(context);
+        // Context has this useful extension to read the body as a string.
+        // This will bake the body contents into the exception message to make debugging easier.
+        var body = context.ReadBodyAsString();
         if (!body.Contains(Text))
         {
             // Add the failure message to the exception. This exception only
             // gets thrown if there are failures.
-            ex.Add($"Expected text '{Text}' was not found in the response body");
+            context.AddFailure($"Expected text '{Text}' was not found in the response body");
         }
     }
 }
 ```
-<sup><a href='https://github.com/JasperFx/alba/blob/master/src/Alba/Assertions/BodyContainsAssertion.cs#L5-L26' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_bodycontainsassertion' title='Start of snippet'>anchor</a></sup>
+<sup><a href='https://github.com/JasperFx/alba/blob/master/src/Alba/Assertions/BodyContainsAssertion.cs#L5-L28' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_bodycontainsassertion' title='Start of snippet'>anchor</a></sup>
 <!-- endSnippet -->
 
 Once you have your assertion class, you can apply it to a scenario through an extension method against the 

src/Alba/Assertions/BodyContainsAssertion.cs

@@ -14,6 +14,8 @@ public BodyContainsAssertion(string text)
 
     public void Assert(Scenario scenario, AssertionContext context)
     {
+        // Context has this useful extension to read the body as a string.
+        // This will bake the body contents into the exception message to make debugging easier.
         var body = context.ReadBodyAsString();
         if (!body.Contains(Text))
         {