Skip with warn for circular symlinks

[ath0mas]

Skip with warn for circular symlinks, instead of exit with error.

Fix #168

[paulmillr]

Why does this require a change to warn? Can't you just handle error instead?

[ath0mas]

Current error means _onError will call destroy and abort the stream.

no more entries, warning or errors are emitted

while my use case is to skip and let the stream continue flowing.
Also, thrown error does not have a code to match it properly.

With this PR it emits a warn event, and indeed with given code someone can use it to abort or not.

// Optionally call stream.destroy() in `warn()` ...