Improve error message when COORDINATOR_DATA_DIRECTORY is unset

This commit updates gpstop and gpstart to use shared logic when
determining the coordinator data directory.

By relying on the common helper, users receive clearer and more
actionable error messages when COORDINATOR_DATA_DIRECTORY is unset
or invalid, making startup and shutdown failures easier to diagnose.

Author: water32

gpMgmt/bin/gpstart

@@ -902,9 +902,7 @@ class GpStart:
 
         if is_external_fts:
             if options.fts_hosts is None:
-                coordinator_data_directory = os.getenv('COORDINATOR_DATA_DIRECTORY')
-                if coordinator_data_directory is None:
-                    coordinator_data_directory = options.coordinatorDataDirectory
+                coordinator_data_directory = gp.get_coordinatordatadir()
                 options.fts_hosts = coordinator_data_directory + '/config' + '/fts_host'
 
         return GpStart(options.specialMode, options.restricted,

gpMgmt/bin/gpstop

@@ -959,9 +959,7 @@ class GpStop:
 
         if is_external_fts:
             if options.fts_hosts is None:
-                coordinator_data_directory = os.getenv('COORDINATOR_DATA_DIRECTORY')
-                if coordinator_data_directory is None:
-                    coordinator_data_directory = options.coordinatorDataDirectory
+                coordinator_data_directory = gp.get_coordinatordatadir()
 
                 options.fts_hosts = coordinator_data_directory + '/config' + '/fts_host'