Started to create different error codes for different backend messages.

src/interfaces/ecpg/ChangeLog

src/interfaces/ecpg/ecpglib/connect.c

src/interfaces/ecpg/ecpglib/error.c

src/interfaces/ecpg/ecpglib/execute.c

src/interfaces/ecpg/ecpglib/extern.h

src/interfaces/ecpg/include/ecpgerrno.h

src/interfaces/ecpg/test/test_informix.pgc