LibWeb: Mark most stream callbacks as infallible

There are a number of script-provided stream callbacks for various
stream operations, such as `start`, `pull`, `cancel`, etc. Out of all of
these, only the `start` callback can actually throw. And when it does,
the exception is realized immediately in the corresponding stream
constructor.

All other callbacks have spec text of the form:

    Throwing an exception is treated the same as returning a rejected
    promise.

And indeed this is internally handled by the streams spec. Thus all of
those callbacks can be specified as returning only a promise, rather
than a WebIDL::ExceptionOr<Promise>.

Userland/Libraries/LibWeb/Streams/ReadableByteStreamController.cpp

@@ -168,7 +168,9 @@ WebIDL::ExceptionOr<void> ReadableByteStreamController::pull_steps(JS::NonnullGC
     readable_stream_add_read_request(*m_stream, read_request);
 
     // 7. Perform ! ReadableByteStreamControllerCallPullIfNeeded(this).
-    return readable_byte_stream_controller_call_pull_if_needed(*this);
+    readable_byte_stream_controller_call_pull_if_needed(*this);
+
+    return {};
 }
 
 // https://streams.spec.whatwg.org/#rbs-controller-private-pull