make LimitSizeFS more robust

fs/base.py

@@ -971,6 +971,9 @@ class FS(object):
             try:
                 src_file = self.open(src, "rb")                
                 self.setcontents(dst, src_file, chunk_size=chunk_size)                
+            except ResourceNotFoundError:
+                if self.exists(src) and not self.exists(dirname(dst)):
+                    raise ParentDirectoryMissingError(dst)
             finally:
                 if src_file is not None:
                     src_file.close()                

fs/tests/__init__.py

@@ -788,7 +788,7 @@ class FSTestCases(object):
             
 
     def test_big_file(self):
-        """Test big files (1MB)"""      
+        """Test handling of a big file (1MB)"""      
         chunk_size = 1024 * 256
         num_chunks = 4
         def chunk_stream():