send-email: finer-grained SMTP error handling

send-email: finer-grained SMTP error handling

Code captured errors but did not process them further.
This treated all failures the same without distinguishing SMTP status.

Add handle-smtp_error to extract SMTP status codes using a regex (as
defined in RFC 5321) and handle errors as follows:

- No error present:
	- If a result is provided, return 1 to indicate success.
	- Otherwise, return 0 to indicate failure.

- Error present with a captured three-digit status code:
	- For 4yz (transient errors), return 1 and allow retries.
	- For 5yz (permanent errors), return 0 to indicate failure.
	- For any other recognized status code, return 1, treating it as
	a transient error.

- Error present but no status code found:
	- Return 1 as a transient error.

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