Standards or Best Practices for Error Handling in SSRS
As billinkc noted in his comment there is no way to propagate the subreport errors to the main reports.

As evidenced in this MSDN post you could try to use data driven subscriptions to drive the subscription, but that could get messy if there is any complexity involved real quick.

If you look at the SQL Agent jobs for a server using SSRS subscriptions you will see a bunch of SQL Agent jobs with a GUID as name, you could modify those jobs to include an extra step running some data validation steps before actually running the report failing the job before the report is sent out (risking your job definition being overwritten by SSRS) or calling the job from another job after validating the data logic.

You could identify the GUID associated with the report using this query:

SELECT
            b.name AS JobName
            , e.name
            , e.path
            , d.description
            , a.SubscriptionID
            , laststatus
            , eventtype
            , LastRunTime
            , date_created
            , date_modified
    FROM ReportServer.dbo.ReportSchedule a JOIN msdb.dbo.sysjobs b
          ON a.ScheduleID = b.name
            JOIN ReportServer.dbo.ReportSchedule c
            ON b.name = c.ScheduleID
            JOIN ReportServer.dbo.Subscriptions d
            ON c.SubscriptionID = d.SubscriptionID
            JOIN ReportServer.dbo.Catalog e
            ON d.report_oid = e.itemid
    WHERE e.name = 'Sales_Report'


(taken from here)

TL/DR: there is no clean solution but you could get away with some hacks, you would need to implement logic in a data driven subscription or create an extra job/modify the generated job

PS: I didn't test any of these approaches