Enable error handling when streaming

[paulmelnikow]

See discussion of the problem in #256 (and a bit more in #548).

Adding proc to the .stream() callback seems like the easiest way to allow the caller to handle this. Tested in a downstream package and it works, though it's not terribly convenient.

When no callback is provided, emit an error event on the throughStream.

Before and after this change, I have two failing tests, which appear to be unrelated.

[paulmelnikow]

No change in behavior. This is a documentation fix.

[paulmelnikow]

Not strictly necessary, but seems useful. ^^

[paulmelnikow]

Moved this block for clarity. It is only effective when bufferOutput is true. See discussion in #256.

[pmoleri]

Makes sense. Just, to explain for the next reviewer:

• there's code to prevent the callback to be called more than once
• when !buffer case calls the callback immediately

So, when !buffer this error would be swallowed by the code that prevents the callback to be called more than once.

[paulmelnikow]

Same errors are appearing in CI. Again, I don't think they are related to this change.

[paulmelnikow]

@aheckmann Could you take a look, please? Would be great to get this fixed for badges/shields#914.

[pmoleri]

Hi @aheckmann , this is an important PR, otherwise the use of .stream() is unreliable.
I guess the PR would have to get rebased, if that's done, would you merge it?
Thanks.

[jonlil]

Hello, I bumped into this in one of my projects today and I more or less solved it by bypassing .stream() like the following.
And if somebody else stumbles onto this here is one solution to at least notice the warnings

async function gmToBuffer (data) {
  return new Promise((resolve, reject) => {
    let chunks = [];
    data._preprocess((err) => {
      if (err) return reject(err);

      data._spawn(data.args(), false, (err, stdout, stderr) => {
        if (err) return reject(err);

        stdout.on('data', (chunk) => chunks.push(chunk));
        stdout.once('end', () => resolve(Buffer.concat(chunks)));
        stderr.on('data', (msg) => {
          const error_message = String(msg);
          if (error_message.includes('@ warning/')) {
            logger.error(error_message);
          } else {
            stdout.emit('error', new Error(error_message));
          }
        });
        stdout.once('error', (err) => reject(err));
      });
    });
  });
}

And to be specific I wanted to still get the warning and not just silence them.