returns stderr too; starting FileSystem stuff

pyadb/internal/__init__.py

@@ -0,0 +1,5 @@
+from pyadb.internal.cli_wrap import AdbWrapper
+
+
+class FileSystem(AdbWrapper):
+    pass

pyadb/internal/cli_wrap.py

@@ -22,7 +22,7 @@ class AdbWrapper:
 
         if output_streams:
             res = subprocess.run(comspec, capture_output=True, check=True)
-            return res.stdout
+            return res.stdout, res.stderr
 
         else:
             ret = subprocess.check_call(comspec, shell=False)