ignore_failed_copy should maybe be removed and replaced by stop_on_error

[fviard]

There is the configuration entry:
"ignore_failed_copy"
that is used to continue processing a "s3cmd cp" / "s3cmd modify" / "s3cmd mv" ...
even when some src files doesn't exist in the s3 server "NoSuchKey".

It should certainly be removed and its use replaced by the new configuration "stop_on_error".
In that case the behavior of these functions will be inversed regarding when "ignore_failed_copy" was the default. But I think that it is not such a big deal for these functions in order to have consistency everywhere.

By the way, "subcmd_cp_mv" should probably be modified to return "EX_PARTIAL" if we are in the continue on error mode and we encounter one or more cases of NoSuchKey.

[fviard]

@Tim-Anderson Can you have a look at that one?
(btw, maybe you can also add a general short section about the stop-on-error option in the README.md file)

[fviard]

Closed already fixed issue!