core: introduce CoreCriticalException. When this one is fired while trying to use a core lib, then the exception is raised to the user.

This is actually used for window provider, while trying to create the window.
+ if an import error is detected, just warn the user about a missing module.

kivy/core/__init__.py

@@ -22,6 +22,9 @@ import os
 import kivy
 from kivy.logger import Logger
 
+class CoreCriticalException(Exception):
+    pass
+
 if 'KIVY_DOC' in os.environ:
     # stub for sphinx generation
     def core_select_lib(category, llist, create_instance=False):
@@ -55,10 +58,24 @@ else:
                     cls = cls()
                 return cls
 
+            except ImportError as e:
+                Logger.warning('%s: Unable to use <%s> as %s'
+                     'provider' % (category.capitalize(), option, category))
+                Logger.warning('%s: Associated module are missing' %
+                        (category.capitalize()))
+                Logger.debug('', exc_info=e)
+
+            except CoreCriticalException as e:
+                Logger.error('%s: Unable to use <%s> as %s'
+                     'provider' % (category.capitalize(), option, category))
+                Logger.error('%s: The module raised an important error' %
+                        (category.capitalize()))
+                raise
+
             except Exception as e:
                 Logger.warning('%s: Unable to use <%s> as %s'
                      'provider' % (category.capitalize(), option, category))
-                Logger.debug('', exc_info = e)
+                Logger.debug('', exc_info=e)
 
         Logger.critical('%s: Unable to find any valuable %s provider '
               'at all!' % (category.capitalize(), category.capitalize()))

kivy/core/window/window_pygame.py

@@ -4,8 +4,11 @@ Window Pygame: windowing provider based on Pygame
 
 __all__ = ('WindowPygame', )
 
-from . import WindowBase
+# fail early if possible
+import pygame
 
+from . import WindowBase
+from kivy.core import CoreCriticalException
 import os
 import sys
 from os.path import exists
@@ -15,12 +18,6 @@ from kivy.logger import Logger
 from kivy.base import stopTouchApp, EventLoop
 from kivy.clock import Clock
 
-try:
-    import pygame
-except:
-    Logger.warning('WinPygame: Pygame is not installed !')
-    raise
-
 # late binding
 glReadPixels = GL_RGB = GL_UNSIGNED_BYTE = None
 
@@ -45,7 +42,10 @@ class WindowPygame(WindowBase):
         if sys.platform == 'linux2':
             self.flags |= pygame.RESIZABLE
 
-        pygame.display.init()
+        try:
+            pygame.display.init()
+        except pygame.error, e:
+            raise CoreCriticalException(e.message)
 
         multisamples = Config.getint('graphics', 'multisamples')