Gerard Mortier, new general manager and artistic director of City Opera, who had joined two years ago with great fanfare, and had promised to revolutionize and revitalize opera in New York, has announced his resignation over deep financial problems at the company, saying it is not possible for him to run the company in the current economic climate.  Departing barely after two years, the French born impresario brought impressive credentials to the job, including a successful run at the Paris Opera.