Prevent phantom books from crashing queries

This is a partial fix for #57

gutenberg/query/api.py

@@ -9,6 +9,7 @@
 from six import with_metaclass
 from rdflib.term import URIRef
 
+from gutenberg._domain_model.exceptions import InvalidEtextIdException
 from gutenberg._domain_model.exceptions import UnsupportedFeatureException
 from gutenberg._domain_model.types import validate_etextno
 from gutenberg._util.abc import abstractclassmethod
@@ -114,7 +115,10 @@ def _uri_to_etext(cls, uri_ref):
         meta-data RDF graph to a human-friendly integer text identifier.
 
         """
-        return validate_etextno(int(os.path.basename(uri_ref.toPython())))
+        try:
+            return validate_etextno(int(os.path.basename(uri_ref.toPython())))
+        except InvalidEtextIdException:
+            return None
 
     @staticmethod
     def __find_implementations():

gutenberg/query/extractors.py

@@ -40,7 +40,8 @@ def get_metadata(cls, etextno):
     @classmethod
     def get_etexts(cls, requested_value):
         query = cls._metadata()[:cls.predicate():cls.contains(requested_value)]
-        return frozenset(cls._uri_to_etext(result) for result in query)
+        results = (cls._uri_to_etext(result) for result in query)
+        return frozenset(result for result in results if result is not None)
 
 
 class AuthorExtractor(_SimplePredicateRelationshipExtractor):