TLS support in MoeNavigatorEngine is currently done via the NetworkHandler_GnuTLS class. It acts like any other network handler, but encrypts data before transferring them and decrypts received data.
To check if a server certificate is valid, the operating system's list of known certificates is used. In case there are errors, a special exception for TLS errors is thrown, called TLSException.
Code that catches TLSException instances can just print out the error message to the user or check the error code of the exception to find out why the exception has been thrown. There are several error codes defined for TLS errors. These are written down in the source code file Exceptions/TLSException.h.