MINOR: connection: add new sample fetch functions fc_err_name and bc_err_name

MINOR: connection: add new sample fetch functions fc_err_name and bc_err_name

These functions return a symbolic error code such as ECONNRESET to keep
logs compact while making them human-readable. It's a good alternative
to the numeric code in that it's more expressive, and a good one to the
full message since it's shorter and more precise (some codes even match
errno names).

The doc was updated so that the symbolic names appear in the table. It
could be useful to backport this feature to help with troubleshooting
some issues, though backporting the doc might possibly be more annoying
in case users have local patches already, so maybe the table update does
not need to be backported in this case.

(cherry picked from commit 601b34fe7bd50c733a437f26817580bbd56c8d56)
Signed-off-by: Willy Tarreau <w@1wt.eu>