Failover when compare tool was not found

[m^2]

If external compare tool is not found, TC shows an error and stops here.
I suggest to show an error and ask whether to run internal one instead.

Normally, in such cases user should fix configuration anyway, but sometimes it's infeasible i.e. when you find 2 conflicting files during a lengthy copying. Currently there's no other way to compare the files than to stop the operation and do it again if the tool can't be started.

And it would just more friendly.