Document the "no such element" error message.

README.rst

@@ -261,12 +261,14 @@ as follows:
   lookup in an associative array.
 
   If ``element`` is set but the lookup fails, that is if there is no
-  such element in the set, then VCL failure is invoked. If the lookup
-  for the ``element`` succeeds, then the successful match establishes
-  a match context for subsequent code. That means that the rules
-  presently described can be applied again, as if ``.match()`` had
-  returned ``true`` for the ``element`` (internally, that is in fact
-  what happens).
+  such element in the set, then VCL failure is invoked, with the
+  string "no such element" in the ``VCL_Error`` log message.
+
+  If the lookup for the ``element`` succeeds, then the successful
+  match establishes a match context for subsequent code. That means
+  that the rules presently described can be applied again, as if
+  ``.match()`` had returned ``true`` for the ``element`` (internally,
+  that is in fact what happens).
 
   The internal match against ``element`` is case sensitive if and only
   if the ``case_sensitive`` flag was ``true`` in the set constructor

src/vmod_selector.vcc

@@ -257,12 +257,14 @@ as follows:
   lookup in an associative array.
 
   If ``element`` is set but the lookup fails, that is if there is no
-  such element in the set, then VCL failure is invoked. If the lookup
-  for the ``element`` succeeds, then the successful match establishes
-  a match context for subsequent code. That means that the rules
-  presently described can be applied again, as if ``.match()`` had
-  returned ``true`` for the ``element`` (internally, that is in fact
-  what happens).
+  such element in the set, then VCL failure is invoked, with the
+  string "no such element" in the ``VCL_Error`` log message.
+
+  If the lookup for the ``element`` succeeds, then the successful
+  match establishes a match context for subsequent code. That means
+  that the rules presently described can be applied again, as if
+  ``.match()`` had returned ``true`` for the ``element`` (internally,
+  that is in fact what happens).
 
   The internal match against ``element`` is case sensitive if and only
   if the ``case_sensitive`` flag was ``true`` in the set constructor