Missing types raise an exception, fix #107

docs/Type/Perl6/Documentable/Primary.pod6

@@ -277,4 +277,17 @@ must follow the format:
 
 =end code
 
+=end pod
+
+=begin pod :kind("type") :subkind("class") :category("exception")
+
+=TITLE class X::Documentable::TypeNotFound
+
+=SUBTITLE Missing type in type-graph file
+
+    X::Documentable::TypeNotFound is Exception {}
+
+Attempt to create the typegraph representation of a missing type. To fix it, add that type
+to the type-graph file you are using.
+
 =end pod

lib/Perl6/Documentable/DocPage/Primary.pm6

@@ -5,6 +5,13 @@ use URI::Escape;
 use Pod::Utilities::Build;
 use Perl6::Documentable;
 
+class X::Documentable::TypeNotFound is Exception {
+    has $.type;
+    method message() {
+        "$.type entry not found in type-graph.txt file."
+    }
+}
+
 class Perl6::Documentable::DocPage::Primary::Language
     does Perl6::Documentable::DocPage {
 
@@ -125,7 +132,8 @@ class Perl6::Documentable::DocPage::Primary::Type
     #| Completes a type pod with inherited routines
     method compose-type($registry, $doc) {
 
-        {return;} unless $registry.tg.types{$doc.name};
+        die X::Documentable::TypeNotFound.new(:type("$doc.name"))
+        unless $registry.tg.types{$doc.name};
 
         $doc.pod.contents.append: self.typegraph-fragment($doc.name);