Return io errors from print_diff instead of panicking on broken pipe

[integral-830]

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

Fixes #7158

OutputWriter::writeln unwrapped the results of writing to stdout, so a closed
pipe turned into a panic (reported on Windows, where BrokenPipe is returned
instead of the process being killed).

writeln and print_diff now return io::Result, and DiffEmitter propagates the
error with ?. The non-terminal path uses writeln! on stdout instead of
println!, which would panic on a failed write too.

I could only test on macOS, so I haven't tried this on Windows.

[ytmimi]

Thank you for the PR.

Marking this as on hold until a minimal reproduction is provided that we can turn into a test case to verify the fix.