Better error for checkdeps.

PRESUBMIT.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Presubmit script for pdfium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
@@ skipping 32 matching lines @@
   # We need to wait until we have an input_api object and use this
   # roundabout construct to import checkdeps because this file is
   # eval-ed and thus doesn't have __file__.
   original_sys_path = sys.path
   try:
     sys.path = sys.path + [input_api.os_path.join(
         input_api.PresubmitLocalPath(), 'buildtools', 'checkdeps')]
     import checkdeps
     from cpp_checker import CppChecker
     from rules import Rule
+  except ImportError:
+    return [output_api.PresubmitError(
+        'Unable to run checkdeps, does pdfium/buildtools/checkdeps exist?')]
   finally:
     # Restore sys.path to what it was before.
     sys.path = original_sys_path
 
   added_includes = []
   for f in input_api.AffectedFiles():
     if not CppChecker.IsCppFile(f.LocalPath()):
       continue
 
     changed_lines = [line for line_num, line in f.ChangedContents()]
@@ skipping 26 matching lines @@
 
 
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results += _CheckUnwantedDependencies(input_api, output_api)
   results += input_api.canned_checks.CheckPatchFormatted(input_api, output_api)
   results += input_api.canned_checks.CheckChangeLintsClean(
       input_api, output_api, None, LINT_FILTERS)
 
   return results