This is not possible to capture the failed syncs, excepted if the failure comes for the sync filter (see "Sync with Marketo" mysteries part 3-How to know a record has stopped sync'ing )
For the rest, I have quite covered here: Marketo / salesforce integration needs a serious overhaul, don't you "sync"? so, just vote for the missing features
The last possibility would be to use the API to extract the logs, analyze them and update back the leads to trigger what you want.
But you probably can capture invalid emails directly in Marketo with a smart list, can't you? then you would use this smart list to avoid syncing them with a choice in you lead management campaign flow step.
We run a campaign and smart list to check such leads and then fix the issues.