Write a method that accepts a character that represents one nitrogenous base in RNA. These bases are encoded according to the following scheme: binary 00='U', binary 01='C', binary 10='A', and binary 11='G'. Return a string that shows the binary representation of the base 'ltr' that is the input parameter. If an invalid 'ltr' is passed in, return the string "error" to indicate that there was an error.