I have a unittest which runs fine on Contentversion exept the .adderror is not covered in my trigger. How do i catch expectedExceptionThrown where as with an system debug i get: DEBUG|link.LinkedEntityId ContentDocumentLink:{LinkedEntityId=0013E00000COrXWQA1, Id=06A3E0000000eVkUAI}. Here is my unittest:

@IsTest(seeAllData=false)
private class TestNoteOnContentversion {
	
    @IsTest 
    static void refuse_shouldAddError_whenOrderregelIsLocked() {
        // arrange
        Account acc = new Account(
        	Name = 'TEST_ACCT', 
            Account_Status_DS__c = 'Status'
        );
        insert acc;
        
        Orderregel__c orderregel = new Orderregel__c(
            Account__c = acc.Id,
            Orderbegindatum__c = Date.today()
        );
        
        insert orderregel;
        
    	ContentVersion content=new ContentVersion(); 
        content.Title='Header_Picture1'; 
        content.PathOnClient='/' + content.Title + '.jpg'; 
        Blob bodyBlob=Blob.valueOf('Unit Test ContentVersion Body'); 
        content.VersionData=bodyBlob; 
        content.origin = 'H';
    	insert content;
    	
        ContentDocumentLink link=new ContentDocumentLink();
        link.LinkedEntityId=acc.id;
        link.contentdocumentid=[select contentdocumentid from contentversion where id =: content.id].contentdocumentid;
        link.ShareType = 'V';
       
    	insert link;
		
        //Id parentId = link.LinkedEntityId;
        
        // Query the Orderregels to lock
        Orderregel__c[] ord = [SELECT Id from Orderregel__c WHERE Account__c  = : acc.Id];
        
  		// Lock the accounts
		Approval.LockResult[] lrList = Approval.lock(ord, false);
        
		Test.startTest();
        try { 
              update content;
            }
        catch (Exception dmx) 
        { 
           Boolean expectedExceptionThrown = dmx.getMessage().contains('link.LinkedEntityId,Id') ? true : false; 
           system.debug(expectedExceptionThrown);
            System.assertEquals(expectedExceptionThrown, false);  
        }
        Test.stopTest();
    }   
}