EAGAIN / EWOULDBLOCK when writing logs to stderr, causing failure

[piegamesde]

Separating out the issue from #367. Backtrace:

Resource temporarily unavailable (os error 11)
Stack backtrace:
0: <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call
1: bupstash::client::SendSession::send_dir
2: bupstash::client::send
3: bupstash::put_main
4: bupstash::main
5: std::sys_common::backtrace::__rust_begin_short_backtrace
6: main

I'll try to get a better trace with line numbers, but this might already be sufficient for tracking it down.

This might very well be an error caused on my side due to how I deploy it (systemd service), but currently I don't understand enough about what's going wrong there to be sure.