Don't crash when providing (by mistake) a directory instead of a file.

[RaphaelMarinier]

Before this commit, running: ./checktestdata <spec_file> would crash with:

terminate called after throwing an instance of 'std::length_error' what(): basic_string::_M_create Aborted (core dumped)

is not expected by checktestdata, and it should be a filename instead. Still, checktestdata should return an error instead of crashing.