Provide 'EEXIST' code for "already exists" errors

[urugator]

So we don't have to match against the error message.

Related: #461

[RyanZim]

Closing for the same reasons as #592.

[urugator]

Can't we pass fs.constants.COPYFILE_EXCL flag to fs.copyFile when overwrite === false and catch/rethrow the native error based on errorOnExist?
It would:

• make the existence check atomic (current check isn't safe in concurrent env)
• provide a way to handle the error meaningfully

[RyanZim]

The problem with that is that we still need to support environments that don't support fs.copyFile.

[urugator]

The fallback uses createWriteStream:

The exclusive flag 'x' (O_EXCL flag in open(2)) ensures that path is newly created. On POSIX systems, path is considered to exist even if it is a symlink to a non-existent file. The exclusive flag may or may not work with network file systems.

Is that viable?

What is the suggested way to handle errorOnExist error?
I mean, the existence check isn't reliable enough as a guarantee for data loss prevention.
But it's also hard to use as a naive "let the user know" check, because the error is "indistinguishable" from others.
So what's the use case? Should the overwrite and errorOnExists option even exist?