source.cmd: reject empty results

also better logging

nvchecker/source/cmd.py

@@ -9,12 +9,22 @@ logger = structlog.get_logger(logger_name=__name__)
 
 async def get_version(name, conf):
   cmd = conf['cmd']
-  p = await asyncio.create_subprocess_shell(cmd, stdout=asyncio.subprocess.PIPE)
+  p = await asyncio.create_subprocess_shell(
+    cmd,
+    stdout=asyncio.subprocess.PIPE,
+    stderr=asyncio.subprocess.PIPE,
+  )
 
-  output = (await p.communicate())[0].strip().decode('latin1')
+  output, error = await p.communicate()
+  output = output.strip().decode('latin1')
+  error = output.strip().decode(errors='replace')
   if p.returncode != 0:
     logger.error('command exited with error',
+                 cmd=cmd, error=error,
                  name=name, returncode=p.returncode)
-    return
-
-  return output
+  elif not output:
+    logger.error('command exited without output',
+                 cmd=cmd, error=error,
+                 name=name, returncode=p.returncode)
+  else:
+    return output