ofp-errors: Ignore text enclosed in square brackets

Enhance to extract-ofp-errors to omit text enclosed in square brackets from
error description. This allows some commentary other than
the error description to be supplied in ofp-errors.h

As suggested by Ben Pfaff <blp@nicira.com>

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com added the large comment on enum ofperr.]
Signed-off-by: Ben Pfaff <blp@nicira.com>