Add some debugging to buildbot_globals, sort the trybot list

tools/buildbot_globals.py

 #!/usr/bin/python
 
 # Copyright (c) 2013 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.
 
 """
 Provides read access to buildbot's global_variables.json .
 """
 
 import json
 import svn
 
 _global_vars = None
 
+
+GLOBAL_VARS_JSON_URL = (
+    'http://skia.googlecode.com/svn/buildbot/site_config/global_variables.json')
+
+
+class GlobalVarsRetrievalError(Exception):
+  """Exception which is raised when the global_variables.json file cannot be
+  retrieved from the Skia buildbot repository."""
+  pass
+
+
+class JsonDecodeError(Exception):
+  """Exception which is raised when the global_variables.json file cannot be
+  interpreted as JSON. This may be due to the file itself being incorrectly
+  formatted or due to an incomplete or corrupted downloaded version of the file.
+  """
+  pass
+
+
 class NoSuchGlobalVariable(KeyError):
+  """Exception which is raised when a given variable is not found in the
+  global_variables.json file."""
   pass
 
+
 def Get(var_name):
   '''Return the value associated with this name in global_variables.json.
   Raises NoSuchGlobalVariable if there is no variable with that name.'''
   global _global_vars
   if not _global_vars:
-    _global_vars = json.loads(svn.Cat('http://skia.googlecode.com/svn/'
-                                      'buildbot/site_config/'
-                                      'global_variables.json'))
+    try:
+      global_vars_text = svn.Cat(GLOBAL_VARS_JSON_URL)
+    except Exception:
+      raise GlobalVarsRetrievalError('Failed to retrieve %s.' %
+                                     GLOBAL_VARS_JSON_URL)
+    try:
+      _global_vars = json.loads(global_vars_text)
+    except ValueError as e:
+      raise JsonDecodeError(e.message + '\n' + global_vars_text)
   try:
     return _global_vars[var_name]['value']
   except KeyError:
     raise NoSuchGlobalVariable(var_name)