BUG/MEDIUM: ssl: improve error processing and reporting in ssl_sock_load_cert_list_file()

BUG/MEDIUM: ssl: improve error processing and reporting in ssl_sock_load_cert_list_file()

fe61656b added the ability to load a list of certificates from a file,
but error control was incomplete and misleading, as some errors such
as missing files were not reported, and errors reported with Alert()
instead of memprintf() were inappropriate and mixed with upper errors.
Also, the code really supports a single SNI filter right now, so let's
correct it and the doc for that, leaving room for later change if needed.