makeparallel: print path on exec failure

If ninja is missing makeparallel prints an unhelpful error:
out/host/linux-x86/bin/makeparallel: exec failed: No such file or directory
which suggests that makeparallel is what is missing, not what is
reporting the error.  Print the path passed to exec as well.

Change-Id: Ic04c1cde6da9c3a974b5c43e3fb3bdb6da6605f7