vcs: collect errors into log

This fixes #39.

nvchecker/source/vcs.py

@@ -1,5 +1,5 @@
 # MIT licensed
-# Copyright (c) 2013-2017 lilydjwg <lilydjwg@gmail.com>, et al.
+# Copyright (c) 2013-2018 lilydjwg <lilydjwg@gmail.com>, et al.
 
 import asyncio
 import os.path as _path
@@ -34,12 +34,19 @@ async def get_version(name, conf, **kwargs):
   cmd = _cmd_prefix + [name, vcs]
   if use_max_tag:
     cmd += ["get_tags"]
-  p = await asyncio.create_subprocess_exec(*cmd, stdout=asyncio.subprocess.PIPE)
+  p = await asyncio.create_subprocess_exec(
+    *cmd,
+    stdout=asyncio.subprocess.PIPE,
+    stderr=asyncio.subprocess.PIPE,
+  )
+
+  output, error = await p.communicate()
+  output = output.strip().decode('latin1')
+  error = error.strip().decode('latin1')
 
-  output = (await p.communicate())[0].strip().decode('latin1')
   if p.returncode != 0:
-    logger.error('command exited with error',
-                 name=name, returncode=p.returncode)
+    logger.error('command exited with error', output=output,
+                 name=name, returncode=p.returncode, error=error)
     return
   else:
     if use_max_tag: