Control over stream writing errors

[weissi]

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.