Report rollbacks to log file
I cant offer you to dump this to the errorlog. For that you need to do some serious research and both these options are lightweight and can leave you with a simple file to look at.

In SQL Server 2005 you need to create a server side trace on transaction filtering on eventsubclass 2. 

In SQL Server 2008R2 and later you are better off creating an extended event session on the 
SQLSERVER.SQL.TRANSACTION event filtering on transaction_state = 2 

But since you are using SQL Server 2005 you need a trace, here is a script that will create a server side trace capturing all rollback events: 

-- Create a Queue
declare @rc int
declare @TraceID int
declare @maxfilesize bigint
set @maxfilesize = 5 

-- Please replace the text InsertFileNameHere, with an appropriate
-- filename prefixed by a path, e.g., c:\MyFolder\MyTrace. The .trc extension
-- will be appended to the filename automatically.

exec @rc = sp_trace_create @TraceID output, 0, N'InsertFileNameHere', @maxfilesize, NULL 
if (@rc != 0) goto error



-- Set the events
declare @on bit
set @on = 1
exec sp_trace_setevent @TraceID, 50, 11, @on
exec sp_trace_setevent @TraceID, 50, 6, @on
exec sp_trace_setevent @TraceID, 50, 8, @on
exec sp_trace_setevent @TraceID, 50, 10, @on
exec sp_trace_setevent @TraceID, 50, 12, @on
exec sp_trace_setevent @TraceID, 50, 13, @on
exec sp_trace_setevent @TraceID, 50, 15, @on
exec sp_trace_setevent @TraceID, 50, 34, @on
exec sp_trace_setevent @TraceID, 50, 35, @on
exec sp_trace_setevent @TraceID, 50, 60, @on
exec sp_trace_setevent @TraceID, 50, 64, @on

-- Set the Filters
declare @intfilter int
declare @bigintfilter bigint
set @intfilter = 2
exec sp_trace_setfilter @TraceID, 21, 0, 0, @intfilter

-- Set the trace status to start
exec sp_trace_setstatus @TraceID, 1

-- display trace id for future references
select TraceID=@TraceID
goto finish

error: 
select ErrorCode=@rc

finish: 
go


For this to work for you, you will need to setup the trace to start with the server. 

For that you need to make sure that the filename is unique each time the trace is started, create a procedure from the trace definition and make it a startup procedure.