fetch2/__init__.py: let try_mirror_url return correct value

The fetcher will try:

1) PREMIRROR
2) Upstream
3) MIRROR

If it fails to download from the Upstream, but succeeds from the MIRROR,
and ud.localpath != origud.localpath (for example, the git tarball),
then we will get the error (e.g.: xf86-video-omapfb):

ERROR: Function failed: Fetcher failure for URL: 'xxx'. Unable to fetch URL from any source.
ERROR: Logfile of failure stored in: /path/to/log.do_fetch.28024

It should not show the error and let the build go on since it succeeds.
(e.g.: xf86-video-omapfb)

[YOCTO #5686]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>