Fix Popen not always returning error code

What does this MR do and why?

Process#waitpid returns Process::Status, which holds a 16-bit value.

• The higher-order 8 bits hold the exit() code (exitstatus).
• The lower-order bits holds whether the process was terminated.

Previously if the process didn't exit normally, Gitlab::Popen#popen would return a status of nil since exitstatus would be nil.

This isn't informative because we want to know what signal killed the process. We can get this by calling Process::Status.to_i.

Relates to #351155 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.