Fix Polyglot.import to raise NameError if not found

* Instead of returning nil unintentionally from Truffle::Interop.lookup_symbol.

Author: eregon

spec/truffle/interop/import_spec.rb

@@ -17,8 +17,10 @@
       Truffle::Interop.import(:imports_an_object).should == object
     end
 
-    it "returns nil because Truffle::Interop.lookup_symbol returns nil if not found" do
-      Truffle::Interop.import(:not_registered_export_test).should == nil
+    it "raises NameError if not found" do
+      -> {
+        Truffle::Interop.import(:not_registered_export_test)
+      }.should raise_error(NameError, "import 'not_registered_export_test' not found")
     end
 
   end

src/main/java/org/truffleruby/RubyLanguage.java

@@ -145,7 +145,12 @@ protected Object findExportedSymbol(RubyContext context, String symbolName, bool
             return null;
         }
 
-        return context.send(context.getCoreLibrary().getTruffleInteropModule(), "lookup_symbol", context.getSymbolTable().getSymbol(symbolName));
+        Object implicit = context.send(context.getCoreLibrary().getTruffleInteropModule(), "lookup_symbol", context.getSymbolTable().getSymbol(symbolName));
+        if (implicit == NotProvided.INSTANCE) {
+            return null;
+        } else {
+            return implicit;
+        }
     }
 
     @SuppressWarnings("deprecation")

src/main/ruby/core/truffle/interop.rb

@@ -113,7 +113,7 @@ def self.lookup_symbol(name)
       elsif Truffle::SymbolOperations.is_constant?(name) && Object.const_defined?(name)
         Object.const_get(name)
       else
-        nil
+        undefined
       end
     end