Fixing truncated error output

[Sequoia]

This is a big annoying old bug that's been around forever apparently. On some systems (mac here, apparently affects windows as well), the following will not work as expected:

console.log(veryLongString);
process.exit();

Console.log is sync but output buffering (writing to stdout stream) is async, so:

• Tick + 0 first chunk is written
• Tick + 1 process.exit()
• Tick + 2 second chunk is written ❌ Never runs

The solution is to pass process.exit as a callback to process.stderr (where console.error goes).

Before

$ markdownlint _includes/readmes/loopback-example-access-control.md
_includes/readmes/loopback-example-access-control.md: 1: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 22: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 24: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 31: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 39: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 41: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 43: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 54: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 56: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 97: MD018 No space after h%
$

After

$ markdownlint _includes/readmes/loopback-example-access-control.md
_includes/readmes/loopback-example-access-control.md: 1: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 22: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 24: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 31: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 39: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 41: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 43: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 54: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 56: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 97: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 105: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 107: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 131: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 146: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 163: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 174: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 180: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 189: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 193: MD018 No space after hash on atx style header
_includes/readmes/loopback-example-access-control.md: 239: MD018 No space after hash on atx style header%
$