update_layer: improve exception handling

When an exception occurs during the main part of update_layer, we were
catching and printing it but that's not enough - we need to do the
following as well:

* Use logger.error() to print the exception information, so that it gets
  logged and highlighted as an error in the layer update
* Exit with a non-zero return code so that update.py knows it has failed

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

layerindex/update_layer.py

@@ -808,7 +808,8 @@ def main():
         pass
     except:
         import traceback
-        traceback.print_exc()
+        logger.error(traceback.format_exc().rstrip())
+        sys.exit(1)
     finally:
         if tinfoil and (LooseVersion(bb.__version__) > LooseVersion("1.27")):
             tinfoil.shutdown()