ofp-parse: Do not exit() upon a parse error.

ofp-parse: Do not exit() upon a parse error.

Until now, failure to parse a flow in the ofp-parse module has caused the
program to abort immediately with a fatal error.  This makes it hard to
use these functions from any long-lived program.  This commit fixes the
problem.

Signed-off-by: Ben Pfaff <blp@nicira.com>