In this patch, I went through any place in DOM fetch code, where there are
ReadableStreams and update the locked, disturbed, readable checks.
Because we expose streams more often, we need an extra care in the use of
ErrorResult objects. JS streams can now throw exceptions and we need to handle
them.
This patch also fixes a bug in FileStreamReader::CloseAndRelease() which could
be called in case mReader creation fails.
