Please make the output of Start-SPMTMigration an object
The output of Start-SPMTMigration is in sentence form, and while you can capture it, you still have to parse it, which is exactly what you're NOT supposed to have to do in PowerShell.
Instead the Output of Start-SPMTMigration should be something like this
GUID Status Reason
efd127ce-68f4-4d85-960d-5384ba561bef Failed The Target Site <URL> does not exist
I mean, you still have to parse the Reason, if you want, but at least you have fewer hoops to jump through just to remove tasks that are destined to fail from your migration.
JB Lewis commented
I realize that Get-SPMTMigration gives the info I think Start-SPMT migration should give, but that should make improving the output of Start-SPMTMigration that much easier since it should just output the exact same object.