Improve error messages in art::GraphChecker and art::SSAChecker

- Add an art::GraphChecker::AddError helper.
- Use StringPrintf instead of std::stringstream.
- Rephrase some error messages.

Change-Id: Ia741e9e67cb5122f086a7383a2bc02d60ca637df
diff --git a/runtime/primitive.h b/runtime/primitive.h
index afcc64d..50d171c 100644
--- a/runtime/primitive.h
+++ b/runtime/primitive.h
@@ -146,6 +146,8 @@
     }
   }
 
+  static const char* PrettyDescriptor(Type type);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(Primitive);
 };