Show actual error message when saving alert rule fails

### Problem Statement

When creating an issue alert rule, if there’s a configuration issue - for example, the ID of the Slack channel [doesn't match with the name]() - the UI only displays a generic “An error occurred” banner when clicking Save.

However, the actual cause (e.g., “Slack: Received channel name {{ channel }} does not match inputted channel name {{ channel }}”) is displayed much higher on the page, requiring the user to scroll up to notice it. This makes it easy to miss the real reason for failure and creates unnecessary confusion.

### Solution Brainstorm

*No response*

### Product Area

Unknown