Fix ZipDirectoryReader under Hiphop

Fix ZipDirectoryReader under Hiphop

ZipDirectoryReaderError subclasses Exception, and sets the code property as a
string. Php's documentation says this should be an int, so this is already a
little bit weird.

Under Hiphop, calling the parent constructor without the second parameter will
set the code property to 0, because the parameter defaults to 0. This leads to
uploads breaking, because calls to getErrorCode return 0 rather than the code
that was passed in.

The change renames the code property to errorCode, so it can't be confused with
exception's code property, which should be an int.

https://bugzilla.wikimedia.org/show_bug.cgi?id=39346

Patch by Chris Keeline <ckeeline@fb.com>

Change-Id: I9b07a5f7f8ba7c980c0cb0da7b65816dc3b97c4c