Reduce md5sum cache write failure log level from error to warning

This is a non-fatal issue that occurs during concurrent project close
when multiple nodes try to write their md5sum cache while the project
directory is being cleaned up.

gns3server/utils/images.py

@@ -290,7 +290,7 @@ def md5sum(path, working_dir=None, stopped_event=None, cache_to_md5file=True):
             with open(md5sum_file, "w+") as f:
                 f.write(digest)
         except OSError as e:
-            log.error("Can't write digest of %s: %s", path, str(e))
+            log.warning("Can't write digest of %s: %s", path, str(e))
 
     return digest