fetcher2: show warning message when checksum is mismatch

Currently, if checksum mismatch, fetcher will try mirror,
and if mirror fetching fail, it will show error "Unable
to fetch URL xxx from from any source" which actually
hide the real reason and make user confuse.

so showing warning message will make it more clear.

Fix [YOCTO #1256]

(Bitbake rev: b3e924297d670963714343d02f7898798fec84fd)

Signed-off-by: Yu Ke <ke.yu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/fetch2/__init__.py

@@ -956,7 +956,7 @@ class Fetch(object):
                         raise
 
                     except BBFetchException as e:
-                        logger.debug(1, str(e))
+                        logger.warn(str(e))
                         # Remove any incomplete fetch
                         if os.path.isfile(ud.localpath):
                             bb.utils.remove(ud.localpath)