Handle conversion failures in 'upon settling'.

Fixes #782.

Author: Ms2ger

Diff for: index.bs

@@ -8236,8 +8236,15 @@ IDL [=promise type=] values are represented by ECMAScript [=PromiseCapability=]
     steps:
 
     1.  Let |onFulfilledSteps| be the following steps given argument |V|:
-        1.  Let |value| be the result of [=converted to an IDL value|converting=] |V| to an IDL
-            value of type |T|.
+        1.  Try running the following step:
+            1.  Let |value| be the result of [=converted to an IDL value|converting=] |V| to an IDL
+                value of type |T|.
+
+            And then, if <a lt="an exception was thrown">an exception |exception| was thrown</a>:
+
+            1.  If there are steps that are required to be run if the promise was rejected, let
+                |result| be the result of performing those steps, given |exception|.
+            1.  [=ECMAScript/throw=] |exception|.
         1.  If there are no steps that are required to be run if the promise was fulfilled, then
             return <emu-val>undefined</emu-val>.
         1.  Let |result| be the result of performing any steps that were required to be run if