Tidy up precondition checking in ErrorReadableStream

Following the established pattern, public API methods should throw if preconditions are violated, whereas abstract operations should assert that they are not.

Author: domenic

index.bs

@@ -559,6 +559,7 @@ Instances of <code>ReadableStreamController</code> are created with the internal
 
 <pre is="emu-alg">
   1. If IsReadableStreamController(*this*) is *false*, throw a *TypeError* exception.
+  1. If *this*@[[controlledReadableStream]]@[[state]] is not "readable", throw a *TypeError* exception.
   1. Return ErrorReadableStream(*this*@[[controlledReadableStream]]).
 </pre>
 
@@ -841,8 +842,9 @@ Instances of <code>ReadableStreamReader</code> are created with the internal slo
 
 <h4 id="error-readable-stream" aoid="ErrorReadableStream">ErrorReadableStream ( stream, e )</h4>
 
+
 <pre is="emu-alg">
-  1. If _stream_@[[state]] is not "readable", throw a *TypeError* exception.
+  1. Assert: _stream_@[[state]] is "readable".
   1. Let _stream_@[[queue]] be a new empty List.
   1. Set _stream_@[[storedError]] to _e_.
   1. Set _stream_@[[state]] to "errored".

reference-implementation/lib/readable-stream.js

@@ -198,6 +198,10 @@ class ReadableStreamController {
       throw new TypeError('ReadableStreamController.prototype.error can only be used on a ReadableStreamController');
     }
 
+    if (this._controlledReadableStream._state !== 'readable') {
+      throw new TypeError(`The stream is ${this._controlledReadableStream._state} and so cannot be errored`);
+    }
+
     return ErrorReadableStream(this._controlledReadableStream, e);
   }
 }
@@ -425,9 +429,7 @@ function EnqueueInReadableStream(stream, chunk) {
 }
 
 function ErrorReadableStream(stream, e) {
-  if (stream._state !== 'readable') {
-    throw new TypeError(`The stream is ${stream._state} and so cannot be errored`);
-  }
+  assert(stream._state === 'readable');
 
   stream._queue = [];
   stream._storedError = e;