[Ada] Fix error handling of Create_Task when thread creation failed

[Arnaud Charlet]

> Did you serioulsy add a fork-bomb to the gcc testsuite with that??  Because
> this test times out for me and in a parallel make check I see spurious
> 
> sh: fork: Resource temporarily unavailable^M
> ...
> /space/rguenther/obj/gcc/as: fork: Resource temporarily unavailable

Well, I indeed hesitated adding this test, although we have no trouble
running this test in our test suites at AdaCore and it;'s a nice stress
test, but if it is causing troubles in some environment and/or if the gcc
test suite is not prepared to handle stress tests, I have no trouble removing
it, let me know.

Arno