Rethrow MissingDataException when needed

In core/document.js: `PDFDocument.prototype.parse` accesses a dictionary
property, which could throw if the underlying data is not yet available.

In core/obj.js: `get Catalog.prototype.metadata` calls
`stream.getBytes`, which can throw MissingDataException too when the
stream is a ChunkedStream.

src/core/obj.js

@@ -104,6 +104,9 @@ var Catalog = (function CatalogClosure() {
           try {
             metadata = stringToUTF8String(bytesToString(stream.getBytes()));
           } catch (e) {
+            if (e instanceof MissingDataException) {
+              throw e;
+            }
             info('Skipping invalid metadata.');
           }
         }