imap-send: improve error messages with configuration hints

imap-send: improve error messages with configuration hints

Replace basic error messages with more helpful ones that guide users
on how to resolve configuration issues. When imap.host or imap.folder
are missing, provide the exact git config commands needed to fix the
problem, along with examples of typical values.

Use the advise() API to display hints in a multi-line format with
proper "hint:" prefixes for each line.

Signed-off-by: Jörg Thalheim <joerg@thalheim.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>