TB-1378 - Fix the overlapping error codes inside libetpan

Set MAILIMAP_ERROR_* codes to start at 1000, so that they don't
overlap with MAIL_ERROR_*.  Both these codes are coming out the
MailCore interface, so the overlapping code ranges is causing problems
diagnosing errors.

Notably, MAIL_ERROR_FETCH and MAILIMAP_ERROR_LOGIN were both 26.