[Omake] "Rule failed to build its targets" needed for cached rules as well?

[Aleksey Nogin]

Consider the following OMakefile:

foo(arg) =
     printvln($@)
     touch $@

a b:
     foo(.)

Now, call "omake a b" and you'll see that only one of the two target was 
actually created (which is kind of expected), but omake fails to 
complain about that! Should we care?

I've added test/targets/Test3 demontstrating this.

Aleksey