Adding a sanity test in 'exists(path)' function

[ntma]

Reason

There is at least one error type (ECONNRESET) in request that causes responses to be null. This will cause an uncaught error in the following line whenever this error occurs:

npm-WebDAV-Client/src/index.ts

Line 287 in ba7280b

callback(e, res.statusCode <= 400);

Fix

Tests if any error is returned by the request before accessing the statusCode of the response.

Warning

This PR changes the signature of the callback in this function from: (error : Error, exists : boolean) to (error : Error, exists ?: boolean). Which means that if an error occurs, the boolean will not be returned!!!

closes #14