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/document.js

@@ -467,6 +467,9 @@ var PDFDocument = (function PDFDocumentClosure() {
           }
         }
       } catch (ex) {
+        if (ex instanceof MissingDataException) {
+          throw ex;
+        }
         info('Something wrong with AcroForm entry');
         this.acroForm = null;
       }