Sender will not raise errors if using thread pool executor

[wk9874]

Description of Bug

IF using the thread pool executor within sender, and one of the threads encounters an error, this error will never be thrown. This is because it is stored in _result, and this is never iterated over to extract any errors.

Steps To Reproduce

Steps to reproduce the behavior:

1. Create a run in offline mode
2. Create a file
3. Do run.save_file()
4. Change the contents of the file
5. Run sender(threading_threshold=1)

Expected behavior

Error should be thrown due to mismatched checksum (#870 )

Actual Behaviour

No error is thrown

Setup

• OS: Ubuntu 22.04
• Python version 3.10

[wk9874]

Fixed in #878