In arv-copy command, fix name error in "except" clause

In arv-copy command, fix name error in "except" clause

In arv_copy.py, when getting or putting (as part of "arv-copy"
operation), the exception-handling clause had undefined names in
themselves.  This would not only throw a hard error at the user but also
prevent subsequent cleanup of the queues.

Fixed by binding the variable name to the exception.

Arvados-DCO-1.1-Signed-off-by: Zoë Ma <zoe.ma@curii.com>