Add extra info to exceptions raised in pipelines. Fixes #407

ResponseErrors generated by commands executed in a pipeline now includes the
command position in the pipeline and the actual command sent to the Redis
server. For example:

Command # 3 (LPUSH c 3) of pipeline caused error: <actual error message from
    Redis server>

1 files changed, 11 insertions, 0 deletions

diff --git a/redis/exceptions.py b/redis/exceptions.py
index d67afa7..0b64f91 100644
--- a/redis/exceptions.py
+++ b/redis/exceptions.py
@@ -1,10 +1,21 @@
 "Core exceptions raised by the Redis client"
+from redis._compat import unicode
 
 
 class RedisError(Exception):
     pass
 
 
+# python 2.5 doesn't implement Exception.__unicode__. Add it here to all
+# our exception types
+if not hasattr(RedisError, '__unicode__'):
+    def __unicode__(self):
+        if isinstance(self.args[0], unicode):
+            return self.args[0]
+        return unicode(self.args[0])
+    RedisError.__unicode__ = __unicode__
+
+
 class AuthenticationError(RedisError):
     pass