{RDBMS} az postgres flexible-server firewall-rule: Update validations and message for fire wall rule commands (#30780)

* Update firewall validations

* Lint fix

Author: nasc17

src/azure-cli/azure/cli/command_modules/rdbms/flexible_server_custom_common.py

@@ -86,6 +86,8 @@ def firewall_rule_create_func(cmd, client, resource_group_name, server_name, fir
         end_ip_address = start_ip_address
     elif start_ip_address is None and end_ip_address is not None:
         start_ip_address = end_ip_address
+    elif start_ip_address is None and end_ip_address is None:
+        raise CLIError("Incorrect Usage : Need to pass in value for either \'--start-ip-address\' or \'--end-ip-address\'.")
 
     if firewall_rule_name is None:
         now = datetime.now()

src/azure-cli/azure/cli/command_modules/rdbms/validators.py

@@ -841,7 +841,8 @@ def validate_public_access_server(cmd, client, resource_group_name, server_name)
 
     server = server_operations_client.get(resource_group_name, server_name)
     if server.network.public_network_access == 'Disabled':
-        raise ValidationError("Firewall rule operations cannot be requested for a private access enabled server.")
+        raise ValidationError("Firewall rule operations cannot be requested for "
+                              "a server that doesn't have public access enabled.")
 
 
 def _validate_identity(cmd, namespace, identity):