[ServiceContract]
public interface IFileService
{
    [OperationContract]
    [WebInvoke(Method = "GET",
        BodyStyle = WebMessageBodyStyle.Bare,
        ResponseFormat = WebMessageFormat.Json,
        UriTemplate = "/DownloadConfig")]
    Stream Download();
}

[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Required)]
public class GCConfigFileService : IGCConfigFileService
{
    public Stream Download()
    {
        throw new Exception();
    }
}

<location path="FileService.svc">
<system.web>
  <authorization>
    <allow users="*"/>
  </authorization>
</system.web>
</location>
<system.serviceModel>
<client />
<behaviors>
  <serviceBehaviors>
    <behavior name="FileServiceBehavior">
      <serviceMetadata httpGetEnabled="true"/>
      <serviceDebug includeExceptionDetailInFaults="false" />
    </behavior>
  </serviceBehaviors>
  <endpointBehaviors>
    <behavior name="web">
      <webHttp/>
    </behavior>
  </endpointBehaviors>
</behaviors>
<serviceHostingEnvironment aspNetCompatibilityEnabled="true"
  multipleSiteBindingsEnabled="true" />
<services>
  <service name="FileService"
           behaviorConfiguration="FileServiceBehavior">
    <endpoint address=""
              binding="webHttpBinding"
              bindingConfiguration="FileServiceBinding"
              behaviorConfiguration="web"
              contract="IFileService"></endpoint>
  </service>
</services>
<bindings>
  <webHttpBinding>
    <binding
      name="FileServiceBinding"
      maxBufferSize="2147483647"
      maxReceivedMessageSize="2147483647"
      transferMode="Streamed"
      openTimeout="04:01:00"
      receiveTimeout="04:10:00"
      sendTimeout="04:01:00">
      <readerQuotas maxDepth="2147483647"
                    maxStringContentLength="2147483647"
                    maxArrayLength="2147483647"
                    maxBytesPerRead="2147483647"
                    maxNameTableCharCount="2147483647" />
    </binding>
  </webHttpBinding>
</bindings>

throw new WebFaultException<string>("Custom Error Message!", HttpStatusCode.InternalServerError);

class HttpErrorHandler : IErrorHandler
{
   public bool HandleError(Exception error)
   {
      return false;
   }

   public void ProvideFault(Exception error, MessageVersion version, ref Message fault)
   {
      if (fault != null)
      {
         HttpResponseMessageProperty properties = new HttpResponseMessageProperty();
         properties.StatusCode = HttpStatusCode.InternalServerError;
         fault.Properties.Add(HttpResponseMessageProperty.Name, properties);
      }
   }
}

class ErrorBehaviorAttribute : Attribute, IServiceBehavior
{
   Type errorHandlerType;

   public ErrorBehaviorAttribute(Type errorHandlerType)
   {
      this.errorHandlerType = errorHandlerType;
   }

   public void Validate(ServiceDescription description, ServiceHostBase serviceHostBase)
   {
   }

   public void AddBindingParameters(ServiceDescription description, ServiceHostBase serviceHostBase, Collection<ServiceEndpoint> endpoints, BindingParameterCollection parameters)
   {
   }

   public void ApplyDispatchBehavior(ServiceDescription description, ServiceHostBase serviceHostBase)
   {
      IErrorHandler errorHandler;

      errorHandler = (IErrorHandler)Activator.CreateInstance(errorHandlerType);
      foreach (ChannelDispatcherBase channelDispatcherBase in serviceHostBase.ChannelDispatchers)
      {
         ChannelDispatcher channelDispatcher = channelDispatcherBase as ChannelDispatcher;
         channelDispatcher.ErrorHandlers.Add(errorHandler);
      }
   }
}

[ServiceContract]
public interface IService
{
   [OperationContract(Action = "*", ReplyAction = "*")]
   Message Action(Message m);
}

[ErrorBehavior(typeof(HttpErrorHandler))]
public class Service : IService
{
   public Message Action(Message m)
   {
      throw new FaultException("!");
   }
}