Fix formatting of restore from backup text

In !40911 (merged), we added a confirmation step if psql encountered any errors. However, the threads used to read stdout/stderr were not joined, so it was likely that the stdout spew from psql would continue to spew before the process was actually complete. As done in Popen3#capture3, this change waits for those stdout/stderr threads to finish before proceeding.

Screenshot when warning occurs: