fs: fix the error report on fs.link(Sync)

As the Node.js documentation specified:

fs.link(srcpath, dstpath, callback)

But when we run:

> fs.link()

We will get the below:

TypeError: dest path must be a string
    at TypeError (native)
    at Object.fs.link (fs.js:915:11)
    at repl:1:4

Just correct the message of relevant 2 errors in this PR :-)