Streams: Consistent behaviour when reading from stream with error.

The streaming operator doesn't short-circuit, consider the following
snippet:

    void foo(InputStream& stream) {
        int a, b;
        stream >> a >> b;
    }

If the first read fails, the second is called regardless. It should be
well defined what happens in this case: nothing.

AK/Stream.h

@@ -75,6 +75,7 @@ namespace AK {
 
 class InputStream : public virtual AK::Detail::Stream {
 public:
+    // Does nothing and returns zero if there is already an error.
     virtual size_t read(Bytes) = 0;
     virtual bool read_or_error(Bytes) = 0;
     virtual bool eof() const = 0;