Add informative python/vpython error message

When running blink web tests it is important to invoke the wrapper
script (run_isolated_script_test.py) with vpython or else psutil and six
will fail to import.

This change wraps the import of psutil and six in an exception handler
so that a useful error message can be printed. This should make it
easier for users to find the correct solution.

Change-Id: Ia2712e97790d8b54bb9cc640ace37a0fe01ce1c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495868
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820453}

testing/test_env.py

@@ -18,7 +18,10 @@ SIX_DIR = os.path.join(
     'six')
 sys.path.insert(0, SIX_DIR)
 
-import six
+try:
+  import six
+except ImportError:
+  raise Exception('Failed to import six. Run under vpython or install six.')
 
 # This is hardcoded to be src/ relative to this script.
 ROOT_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))

testing/xvfb.py

@@ -8,7 +8,6 @@
 
 import os
 import os.path
-import psutil
 import random
 import re
 import signal
@@ -18,6 +17,11 @@ import threading
 import time
 import test_env
 
+try:
+  import psutil
+except ImportError:
+  raise Exception(
+        'Failed to import psutil. Run under vpython or install psutil.')
 
 class _XvfbProcessError(Exception):
   """Exception raised when Xvfb cannot start."""