lockfile: improve error message when lockfile exists

lockfile: improve error message when lockfile exists

A common mistake leading a user to see this message is to launch "git
commit", let the editor open (and forget about it), and try again to
commit.

The previous message was going too quickly to "a git process crashed"
and to the advice "remove the file manually".

This patch modifies the message in two ways: first, it considers that
"another process is running" is the norm, not the exception, and it
explicitly hints the user to look at text editors.

The message is 2 lines longer, but this is not a problem since
experienced users do not see the message often.

Helped-by: Moritz Neeb <lists@moritzneeb.de>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>