fix display hook error message

## What does this MR do?

Fix hook error message display.  

## Are there points in the code the reviewer needs to double check?

Probably Hook will return the plain multi line text.  It is better `pre` tag than `h4`.  
I thought of another good looking display, but I did not come up with it. 😓 

## Why was this MR needed?

When the hook returns an STDERR, "Merge in progress" button spinning forever. Due to javascript's error.  

## Screenshots (if relevant)

When update hook returns an error like

```
XXXXXXXX

YYYYYYYY
ZZZZZZZZ
```

![screen_shot](/uploads/8dac77972a309dfcb72c741dffbd7270/screen_shot.png)

## What are the relevant issue numbers?

Closes #24020

See merge request !7775