better low level handling of missing python installation

Author: crazyeng

wolframclient/cli/commands/start_externalevaluate.py

@@ -1,9 +1,11 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
 import os
+import sys
 
 from wolframclient.cli.utils import SimpleCommand
 from wolframclient.utils.api import externalevaluate as ev
+from wolframclient.utils.api import zmq
 
 
 class Command(SimpleCommand):
@@ -19,4 +21,11 @@ def handle(self, port=None, installpath=None, **opts):
         if installpath:
             os.environ["WOLFRAM_INSTALLATION_DIRECTORY"] = installpath
 
+        try:
+            zmq.Context
+        except ImportError as e:
+            print("%s. Please install zmq. https://zeromq.org/languages/python/" % e, file=sys.stderr)
+            sys.stderr.flush()
+            sys.exit(1)
+
         ev.start_zmq_loop(port=port)