{RDBMS} az postgres flexible-server create/update: change the error message for SKU name validator (#30900)

* {RDBMS} az postgres flexible-server create/update: change the error message for SKU name validator

* {RDBMS} az postgres flexible-server create/update: change the error message for SKU name validator

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Co-authored-by: Matthew Boentoro <mboentoro@microsoft.com>

Author: mattboentoro

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

@@ -472,14 +472,18 @@ def compare_sku_names(sku_1, sku_2):
 
 
 def _pg_sku_name_validator(sku_name, sku_info, tier, instance):
+    additional_error = ''
     if instance is not None:
         tier = instance.sku.tier if tier is None else tier
+    else:
+        additional_error = 'When --tier is not specified, it defaults to GeneralPurpose. '
     if sku_name:
         skus = [item.lower() for item in get_postgres_skus(sku_info, tier.lower())]
         if sku_name.lower() not in skus:
-            raise CLIError('Incorrect value for --sku-name. The SKU name does not match {} tier. '
-                           'Specify --tier if you did not. Or CLI will set GeneralPurpose as the default tier. '
-                           'Allowed values : {}'.format(tier, sorted(skus, key=cmp_to_key(compare_sku_names))))
+            raise CLIError('Incorrect value for --sku-name. The SKU name does not exist in {} tier. {}'
+                           'Provide a valid SKU name for this tier, or specify --tier with the right tier for the '
+                           'SKU name chosen. Allowed values : {}'
+                           .format(tier, additional_error, sorted(skus, key=cmp_to_key(compare_sku_names))))
 
 
 def _pg_storage_performance_tier_validator(performance_tier, sku_info, tier=None, storage_size=None):