generate_token: Give version arg a default value (3).

Also a more useful error message if incorrect.
This makes it works as documented in docs/origin_trials_integration.md

Change-Id: I97d549d10917911d636fc8d41ebd58b1f32838de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415750
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Jason Chase <chasej@chromium.org>
Reviewed-by: Jason Chase <chasej@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807898}

tools/origin_trials/generate_token.py

@@ -142,6 +142,7 @@ def main():
   parser.add_argument("--version",
                       help="Token version to use. Currently only version 2"
                       "and version 3 are supported.",
+                      default='3',
                       type=VersionFromArg)
   parser.add_argument("origin",
                       help="Origin for which to enable the feature. This can "
@@ -214,7 +215,7 @@ def main():
     sys.exit(1)
 
   if (not args.version):
-    print("Invalid token version.")
+    print("Invalid token version. Only version 2 and 3 are supported.")
     sys.exit(1)
 
   if (args.is_third_party is not None and args.version[0] != 3):