gtls: add return checks and optimize the code

gtls: add return checks and optimize the code

This commit does the following things:

1. Update the description of gtls_init()

2. In gtls_client_init(), check the invaild SSLVERSION at first. Note
   that this part refactors the duplicate/incompatible checks and removes
   the useless local variable `sni`.

3. Check the return value of gnutls_ocsp_resp_init(). Although the
   original code is safe because gnutls_ocsp_resp_import() will check
   the validity of `ocsp_resp`, it is better to catch the error in time
   and record the proper message to output log.

Closes #19366