CRM-13079 Add logic to MembershipBlock formRule to verify that the numbe...

...r of enabled membership types is not greater than max allowed in the underlying CRM_Price_Field NUM_OPTION constant. Prevents silent failure. Also bump up the constant to 15 from 11.

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* CRM-13079: hard limit on number of options for price field; no warning/error
  http://issues.civicrm.org/jira/browse/CRM-13079