Use the ESLint no-restricted-syntax rule to ensure that assert is always called with two arguments

Having `assert` calls without a message string isn't very helpful when debugging, and it turns out that it's easy enough to make use of ESLint to enforce better `assert` call-sites.
In a couple of cases the `assert` calls were changed to "regular" throwing of errors instead, since that seemed more appropriate.

Please find additional details about the ESLint rule at https://eslint.org/docs/rules/no-restricted-syntax

src/core/worker_stream.js

@@ -26,7 +26,10 @@ class PDFWorkerStream {
   }
 
   getFullReader() {
-    assert(!this._fullRequestReader);
+    assert(
+      !this._fullRequestReader,
+      "PDFWorkerStream.getFullReader can only be called once."
+    );
     this._fullRequestReader = new PDFWorkerStreamReader(this._msgHandler);
     return this._fullRequestReader;
   }