Control over stream writing errors

AsyncProcess gives some control and information about potential standard input stream writing options even when automatically pulling from an AsyncSequence<Bytes>.

In a way, this is similar to set -o pipefail:

• Sometimes you care about write errors (because you need to make sure that all bytes have been read), similar to set -o pipefail
• Sometimes you don't care
• Sometimes you want to terminate the process on write errors
• Sometimes you don't

We need to double-check that we have the required controls there.

Start by reviewing the linked AsyncProcess.swift ProcessExecutor.swift and ProcessExit.swift sections, then trace how automatic AsyncSequence input handles stream writes. Clarify the desired controls for observing write errors and terminating the process, and confirm the resulting behavior with relevant subprocess tests.