Newlines in custom hook messages are currently not shown in pop up messages

Sample steps to illustrate the current behaviour:

1. Launch a Bitbucket instance
2. Install the plugin Adaptavist ScriptRunner for Bitbucket Server
3. in Administration > ScriptRunner, go to "Pre Hooks"
4. Create a "Custom Pre Hook" script and set the trigger to "branch-delete"
5. Use the following script to throw a popup message during branch deletion

   import com.atlassian.bitbucket.hook.repository.RepositoryHookResult
   
   RepositoryHookResult.rejected("Branch delete", "First line" + "\nSecond line"+ "\r\nThird line" + "\r\n\r\nFourth Line")
   

   In this sample script, newlines were added

6. Locate a repository and delete a branch from the UI

The newlines in the custom message are not shown and are being removed:

The JSON response from the DELETE <BITBUCKET_URL>/rest/branch-utils/latest/projects/<PROJECT_KEY>/repos/<REPO_SLUG>/branches request shows the following:

{
    "errors":
    [
        {
            "context": null,
            "message": "Branch 'test-branch' could not be deleted",
            "exceptionName": "com.atlassian.bitbucket.hook.repository.RepositoryHookVetoedException",
            "details":
            [
                "Branch delete\nFirst line\n\nSecond line\r\nThird line\r\n\r\nFourth Line"
            ],
            "trigger":
            {
                "id": "branch-delete",
                "abortOnFirstVeto": false
            }
        }
    ]
}

The "details" includes the newline chars (e.g. "\n")

But the newlines are removed in the pop up message (highlighted in green):