[llvm-exegesis] Print errno on failures in subprocess

Some error logging in llvm-exegesis under the subprocess executor just
prints a generic failure information rather than any details about the
error as we omit printing the string version of errno. This patch adds
in printing errno at all relevant points in the subprocess executor that
were previously missed.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D157682