Don’t fail fatally if temporary cpp_stderr file is missing

Don’t fail fatally if temporary cpp_stderr file is missing

If the temporary cpp_stderr file is missing when the compilation command
has finished then either some user removed it or ccache removed it in
clean_up_internal_tempdir (since the compilation took more than hour or
the system clock was adjusted?). In either case, let’s just fall back to
running the real compiler instead of failing fatally.

Fixes #480 and #154.