[platform] improve testing assertions

recipe_modules/platform/test_api.py

 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 from recipe_engine import recipe_test_api
 
 class PlatformTestApi(recipe_test_api.RecipeTestApi):
   @recipe_test_api.mod_test_data
   @staticmethod
   def name(name):
-    assert name in ('win', 'linux', 'mac')
+    assert name in ('win', 'linux', 'mac'), 'unknown platform %r' % (name,)
     return name
 
   @recipe_test_api.mod_test_data
   @staticmethod
   def bits(bits):
-    assert bits in (32, 64)
+    assert bits in (32, 64), 'unknown bitness %r' % (bits,)
     return bits
 
   @recipe_test_api.mod_test_data
   @staticmethod
   def cpu_count(cpu_count):
-    assert isinstance(cpu_count, int)
-    assert cpu_count > 0
+    assert isinstance(cpu_count, int), 'bad type %r' % (type(cpu_count),)
+    assert cpu_count > 0, 'bad cpu_count %r' % (cpu_count,)
     return cpu_count
 
   def __call__(self, name, bits):
     return self.name(name) + self.bits(bits)