Add to_owned for Err<Error<&[u8]>>, Err<Error<&str>>
Add to_owned for Err<Error<&[u8]>>, Err<Error<&str>>

When commit 981d036ca8dbc373b8b3c74293491a8c21e18f63 changed the basic
error type, it left no way for the user to take ownership of the
values of this new type.

Fixes #1254.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

Geal/nom