Add missing method.

`CollectionSequenceDataConverter` uses `getValue()` so this message has been
implemented along the other methods for missing values.

src/z3c/form/term.py

@@ -80,7 +80,10 @@ def getTermByToken(self, token):
         raise LookupError(token)
 
     def getValue(self, token):
-        return self.terms.getValue(token)
+        try:
+            return self.terms.getValue(token)
+        except KeyError:
+            raise LookupError(token)
 
     def __iter__(self):
         for value in self.source:
@@ -294,7 +297,21 @@ def getTermByToken(self, token):
                         # fall back on LookupError, otherwise we might accept
                         # any crap coming from the request
                         return term
-            raise LookupError(token)
+            raise
+
+    def getValue(self, token):
+        try:
+            return super(MissingCollectionTermsMixin, self).getValue(token)
+        except LookupError:
+            if self._canQueryCurrentValue():
+                for value in self._queryCurrentValue():
+                    term = self._makeMissingTerm(value)
+                    if term.token == token:
+                        # check if the given token matches the value, if not
+                        # fall back on LookupError, otherwise we might accept
+                        # any crap coming from the request
+                        return value
+            raise
 
 
 class MissingCollectionTermsVocabulary(MissingCollectionTermsMixin,

src/z3c/form/term.txt

@@ -555,6 +555,14 @@ The same goes with looking up a term by value:
   ...
   LookupError: 42
 
+The same goes with looking up a value by the token:
+
+  >>> terms.getValue('42')
+  Traceback (most recent call last):
+  ...
+  LookupError: 42
+
+
 Ooops, well this works only if the context has the right value for us.
 This is because we don't want to accept any crap that's coming from HTML.
 
@@ -598,6 +606,11 @@ Override ``makeMissingTerm`` if you want your own.
   >>> missingTerm.title
   u'Missing: ${value}'
 
+We can get the value for a missing term:
+
+  >>> terms.getValue('42')
+  42
+
 Still we raise LookupError if the token does not fit the context's value:
 
   >>> missingTerm = terms.getTermByToken('99')