NodeCommandError.__str__: Address Python 3.6+

Unify message among all supported Python versions by removing the
trailing period from basic CalledProcessError.__str__ introduced in 3.6

Author: userzimmermann

nodely/error.py

@@ -22,4 +22,4 @@ def __init__(self, returncode, cmd, cwd):
 
     def __str__(self):
         return "{} in working directory {!r}".format(
-            super(NodeCommandError, self).__str__(), self.cwd)
+            super(NodeCommandError, self).__str__().rstrip("."), self.cwd)