tools: Create more consistent checks for venv.

This helps make the Zulip development environment somewhat more robust
to new contributors, since it will give them a nice warning if they
try running any of our development tools outside the Zulip virtualenv.

Fixes #3468.

tools/html-grep

@@ -6,13 +6,12 @@ import optparse
 import sys
 
 from six.moves import filter
-try:
-    import lister
-except ImportError as e:
-    print("ImportError: {}".format(e))
-    print("You need to run the Zulip linters inside a Zulip dev environment.")
-    print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
-    sys.exit(1)
+
+# check for the venv
+from lib import sanity_check
+sanity_check.check_venv(__file__)
+
+import lister
 
 USAGE = '''
     This file greps HTML files for keywords in a context-sensitive manner.