Refactor "mutually exclusive options" error reporting code in parse_subscription_options

Hi,

parse_subscription_options function has some similar code when
throwing errors [with the only difference in the option]. I feel we
could just use a variable for the option and use it in the error.
While this has no benefit at all, it saves some LOC and makes the code
look better with lesser ereport(ERROR statements. PSA patch.

Thoughts?

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com