Show an error message instead of crashing when no C++ constructor is declared.

Closes #1791.

Cython/Compiler/ExprNodes.py

@@ -5157,6 +5157,10 @@ class CallNode(ExprNode):
         elif type and type.is_cpp_class:
             self.args = [ arg.analyse_types(env) for arg in self.args ]
             constructor = type.scope.lookup("<init>")
+            if not constructor:
+                error(self.function.pos, "no constructor found for C++  type '%s'" % self.function.name)
+                self.type = error_type
+                return self
             self.function = RawCNameExprNode(self.function.pos, constructor.type)
             self.function.entry = constructor
             self.function.set_cname(type.empty_declaration_code())