Datasource that outputs text and an error code?

What Mike and Stuart stated, all of the above is true.  Sad but true.  However....what caught my attention - 

is it possible to pass the error message from a variable into the alarm message...in the same way that tokens work.

And the answer is unequivocally yes!  We do it all the time here.  Now, it takes little of bit elbow grease but it works great.
And it is all based on the fact you can use ##<DEVICEPROPERTYNAME>## as one of the DataSource Tokens in your alert message.  Yeap, it is stated so right there on Tokens Available in LogicModule Alert Messages support website.  So then, why not craft your own custom Device Property name that you can pass to the alert message?

In your DataSource you define a customized Alert message for one of the DataPoints and you can pass the value of your device property (ie token) into the Subject and/or Description.  Like this;

Subject:
##LEVEL## - ##HOST## ##DATASOURCE## ##DATAPOINT## ##VALUE##

Description:
This alert will be raised when the reported value of Autoclient State is other than STARTED (value 1). See command output below for detailed description of error.
Possible State values;
1=STARTED   <-- all good
0=STOPPED   <-- alert raised

AutoClient command output:
##MB.SWIFT.AUTOCLIENT.SUMMARY##

Host: ##HOST##
Datasource: ##DATASOURCE##
Datapoint: ##DATAPOINT##
Level: ##LEVEL##
Start: ##START##
Duration: ##DURATION##
Value: ##VALUE##
ClearValue: ##CLEARVALUE##
Reason: ##DATAPOINT## ##THRESHOLD##

Notice I use ##MB.SWIFT.AUTOCLIENT.SUMMARY## custom token that I manipulate in my DataSource to update its value with free text.  The trick is - you need to update this property BEFORE you output whatever DPs you are collecting in your DataSource.

And how do you do that?  Use LM API and /device/devices/{device_id}/properties/{prop_name} endpoint.

Maybe little bit kludgy but again, once you figure out the syntax and workflow in your script it works remarkably well.