Handle exceptions thrown while converting JS return values

Author: jyasskin

index.bs

@@ -14186,7 +14186,8 @@ the special value “missing”, which represents a missing optional argument.
         |callResult| and jump to the step labeled <a href="#call-user-object-operation-return"><i>return</i></a>.
     1.  Set |completion| to the result of [=converted to an IDL value|converting=]
         |callResult|.\[[Value]] to an IDL value of the same type as the operation's
-        return type.
+        return type. If this throws an exception, set |completion| to the completion value
+        representing the thrown exception.
     1.  <i id="call-user-object-operation-return">Return:</i> at this
         point |completion| will be set to an IDL value or an [=abrupt completion=].
         1.  [=Clean up after running a callback=] with |stored settings|.
@@ -14273,8 +14274,9 @@ described in the previous section).
     1.  If |callResult| is an [=abrupt completion=], set |completion| to
         |callResult| and jump to the step labeled <a href="#invoke-return"><i>return</i></a>.
     1.  Set |completion| to the result of [=converted to an IDL value|converting=]
-        |callResult|.\[[Value]] to an IDL value of the same type as the operation's
-        return type.
+        |callResult|.\[[Value]] to an IDL value of the same type as |callable|'s
+        return type. If this throws an exception, set |completion| to the completion value
+        representing the thrown exception.
     1.  <i id="invoke-return">Return:</i> at this
         point |completion| will be set to an IDL value or an [=abrupt completion=].
         1.  [=Clean up after running a callback=] with |stored settings|.
@@ -14315,8 +14317,9 @@ a return type that is a [=promise type=].
     1.  If |callResult| is an [=abrupt completion=], set |completion| to
         |callResult| and jump to the step labeled <a href="#construct-return"><i>return</i></a>.
     1.  Set |completion| to the result of [=converted to an IDL value|converting=]
-        |callResult|.\[[Value]] to an IDL value of the same type as the operation's
-        return type.
+        |callResult|.\[[Value]] to an IDL value of the same type as |callable|'s
+        return type. If this throws an exception, set |completion| to the completion value
+        representing the thrown exception.
     1.  <i id="construct-return">Return:</i> at this
         point |completion| will be set to an IDL value or an [=abrupt completion=].
         1.  [=Clean up after running a callback=] with |stored settings|.