tun-metadata: Provide error messages during auto-allocation.

tun-metadata: Provide error messages during auto-allocation.

In cases where we don't have a map of tunnel metadata options (such
as with ovs-ofctl) we dynamically allocate them as part of the match.
However, dynamic allocation brings the possibility of errors such as
duplicate entries or running out of space. Up until now, anything that
would cause an error was silently ignored. Since that is not very user
friendly, this adds a mechanism for reporting these types of errors.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>