send-email: capture errors in an eval {} block

send-email: capture errors in an eval {} block

Auth relied solely on return values without catching errors. This misjudges
non-credential errors as auth failure without error info.

Patch wraps the entire auth process in an eval {} block to catch
all exceptions, including non-credential errors. It adds a new $error var,
uses 'or do' to prevent flow break, and returns $result ? 1 : 0. And merges
if/else branches, integrates SASL and basic auth, with comments for
future status code handling.

Signed-off-by: Zheng Yuting <05ZYT30@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>