Ignore import errors in cr recursive module loading

tools/cr/cr/loader.py

 # Copyright 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.
 
 """Module scan and load system.
 
 The main interface to this module is the Scan function, which triggers a
 recursive scan of all packages and modules below cr, with modules being
 imported as they are found.
 This allows all the plugins in the system to self register.
@@ skipping 28 matching lines @@
 
 
 def _Import(name):
   """Import a module or package if it is not already imported."""
   module = sys.modules.get(name, None)
   if module is not None:
     return module
   return import_module(name, None)
 
 
+def _TryImport(name):
+  """Try to import a module or package if it is not already imported."""
+  try:
+    return _Import(name)
+  except ImportError:
+    if cr.context.verbose:
+      print 'Warning: Failed to load module', name
+    return None
+
+
 def _ScanModule(module):
   """Runs all the scan_hooks for a module."""
   scanner_tags = getattr(module, _MODULE_SCANNED_TAG, None)
   if scanner_tags is None:
     # First scan, add the scanned marker set.
     scanner_tags = set()
     setattr(module, _MODULE_SCANNED_TAG, scanner_tags)
   for scan in scan_hooks:
     if scan not in scanner_tags:
       scanner_tags.add(scan)
@@ skipping 10 matching lines @@
     except OSError:
       basenames = []
     packages = []
     for basename in basenames:
       fullpath = os.path.join(path, basename)
       if os.path.isdir(fullpath):
         name = '.'.join([package.__name__, basename])
         packages.append(name)
       elif basename.endswith('.py') and not basename.startswith('_'):
         name = '.'.join([package.__name__, basename[:-3]])
-        module = _Import(name)
-        _ScanModule(module)
-        modules.append(module)
+        module = _TryImport(name)
+        if module:
+          _ScanModule(module)
+          modules.append(module)
     for name in packages:
-      child = _Import(name)
-      modules.extend(_ScanPackage(child))
+      child = _TryImport(name)
+      if child:
+        modules.extend(_ScanPackage(child))
   return modules
 
 
 def Import(package, name):
   module = _Import(package + '.' + name)
   path = getattr(module, '__path__', None)
   if path:
     _ScanPackage(module)
   else:
     _ScanModule(module)
@@ skipping 10 matching lines @@
   namespace directly.
 
   Modules are allowed to refer to each other, their import will be retried
   until it succeeds or no progress can be made on any module.
   """
   modules = _ScanPackage(cr)
   # Now scan all the found modules one more time.
   # This happens after all imports, in case any imports register scan hooks.
   for module in modules:
     _ScanModule(module)