more explicit error messages during common import errors

modules/aum/captcha.py

@@ -20,7 +20,11 @@
 
 import hashlib
 import sys
-import Image
+
+try:
+    import Image
+except ImportError:
+    raise ImportError('Please install python-imaging')
 
 class CaptchaError(Exception): pass
 

modules/freemobile/pages/login.py

@@ -19,7 +19,11 @@
 
 
 import time
-import Image
+
+try:
+    import Image
+except ImportError:
+    raise ImportError('Please install python-imaging')
 
 from weboob.tools.browser import BasePage
 

modules/societegenerale/captcha.py

@@ -19,7 +19,11 @@
 
 
 import hashlib
-import Image
+
+try:
+    import Image
+except ImportError:
+    raise ImportError('Please install python-imaging')
 
 
 class TileError(Exception):

modules/youtube/backend.py

@@ -20,8 +20,12 @@
 
 from __future__ import with_statement
 
+try:
+    import gdata.youtube.service
+except ImportError:
+    raise ImportError("Please install python-gdata")
+
 import datetime
-import gdata.youtube.service
 import re
 import urllib