Web job logger implementation

Interface for logger:

 public interface ILogger
        {
            void Log(Exception ex, string contextualMessage = "");

            void Log(string message);
        }

Interface for web job logger:

public interface IJobLogger : ILogger
{
    void SetWriter(TextWriter logWriter);
}

Web job logger:

 public class WebJobLogger : IJobLogger
    {
        private TextWriter writer;

        public WebJobLogger()
        {
            writer = TextWriter.Null;
        }

        public void SetWriter(TextWriter logWriter)
        {
            writer = logWriter;
        }

        public void Log(Exception ex, string contextualMessage = "")
        {
            writer.WriteLine("Logger: Exception thrown with message {0}, exception : {1}", contextualMessage, ex);
        }

        public void Log(string message)
        {
            writer.WriteLine("Logger: {0}", message);
        }
    }

Usage:

public async Task TestJob(
    [QueueTrigger(QueueName.StorageExcelImport)] StartImportCommand blobInfo,
    [Blob(Global.BlobContainerName + BlobReferenceName, FileAccess.Read)] Stream input,
    TextWriter logWriter)
{
    await logWriter.WriteLineAsync("Start");
    try
    {
        //Implemetation
    }
    catch (Exception ex)
    {
        //Set writer
        log.SetWriter(logWriter);
        //Log exception
        log.Log(ex);
    }
    await logWriter.WriteLineAsync(string.Format("File: '{0}' successfuly processed from blob to the database. ({1})", blobInfo.OriginalFileName, DateTime.UtcNow));
}

Dependency injection:

container.Register<ILogger, WebJobLogger>();

The problem with this is that I always have to call log.SetWriter(logWriter); before call log.Log(ex);. If I forget that, then will not work. The whole point of this is to mock logger in unit test. What do you think?