unixctl: Use ovs_retval_to_string() where EOF is a possible value.

unixctl: Use ovs_retval_to_string() where EOF is a possible value.

jsonrpc_transact_block() might return EOF so passing its return value to
strerror() isn't general enough.

It might be better to change jsonrpc_transact{_block}() to never return
EOF, since a closed connection seems like it is always an error in that
context.

Found by Coverity.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>