Fix handling of FormData errors

[bajtos]

This patch is fixing regression introduced by 6670a0f, see #1466 for detailed discussion.

/cc @kornelski @lapo-luchini

I have verified the fix by running the following test manually. Please advise me how to capture these test cases in your automated test suite.

The first two use cases are already captured in the existing test suite, so I updated the existing tests to match the new behavior. Then I added two new tests to verify what happens when two errors are triggered by a single request (the last case below).

// remote host is listening, attaching a missing file
// PROMISE VARIANT
require('.').post('http://example.com/does-not-exist')
.field({a:1,b:2})
.attach('c', 'does-not-exists.txt')
//.on('error', e => console.log('ERR', e.message))
.then(ok => console.log('THEN'))
.catch(e => console.log('CATCH', e.message));

// remote host is listening, attaching a missing file
// CALLBACK VARIANT
require('.').post('http://example.com/does-not-exist')
  .field({a:1,b:2})
  .attach('c', 'does-not-exists.txt')
  //.on('error', e => console.log('ERR', e.message))
  .end((err, res) => {
    if (err) console.log('CATCH', err && err.message);
    else console.log('THEN');
  });

// remote host is not listening
// two errors are triggered: cannot read the file, cannot connect to 127.0.0.1:80
require('.').post('http://127.0.0.1/test')
.field({a:1,b:2})
.attach('c', 'does-not-exists.txt')
//.on('error', e => console.log('ERR', e.message))
.then(ok => console.log('THEN'))
.catch(e => console.log('CATCH', e.message));

[bajtos]

I have added two new tests to verify that ECONNRESET errors take precedence over ENOENT errors, I think we have sufficient test coverage now. I also fixed npm test failures and improved the commit message to better describe what is being changed.

As far as I am concerned, this pull request is ready for final review and merging.

@kornelski Could you please take a look? I understand you might not have time to do it right now, in which case could you please let me know when you will have time to process my pull request?