Prevent browser exceptions from incorrectly triggering the assert in PDFPageProxy._abortOperatorList (PR 11069 follow-up)

For certain canvas-related errors (and probably others), the browser rendering exceptions may be propagated "as-is" to the PDF.js code. In this case, the exceptions are of the somewhat cryptic `NS_ERROR_FAILURE` type.
Unfortunately these aren't actual `Error`s, which thus ends up unintentionally triggering the `assert` in `PDFPageProxy._abortOperatorList`; sorry about that!

src/display/api.js

@@ -1378,7 +1378,8 @@ class PDFPageProxy {
    * @private
    */
   _abortOperatorList({ intentState, reason, force = false, }) {
-    assert(reason instanceof Error,
+    assert(reason instanceof Error ||
+           (typeof reason === 'object' && reason !== null),
            'PDFPageProxy._abortOperatorList: Expected "reason" argument.');
 
     if (!intentState.streamReader) {