Remove most assert() calls (issue 8506)

This replaces `assert` calls with `throw new FormatError()`/`throw new Error()`.
In a few places, throwing an `Error` (which is what `assert` meant) isn't correct since the enclosing function is supposed to return a `Promise`, hence some cases were changed to `Promise.reject(...)` and similarily for `createPromiseCapability` instances.

src/core/worker.js

@@ -541,7 +541,9 @@ var WorkerMessageHandler = {
         if (source.chunkedViewerLoading) {
           pdfStream = new PDFWorkerStream(source, handler);
         } else {
-          assert(PDFNetworkStream, './network module is not loaded');
+          if (!PDFNetworkStream) {
+            throw new Error('./network module is not loaded');
+          }
           pdfStream = new PDFNetworkStream(data);
         }
       } catch (ex) {