truncate potentially long CompositeErrors

If you have a typo in a task on 100 engines, the local exception will be a bit ridiculous.

This defaults the truncation to 4 tracebacks (could be even fewer, or even one?),
and you can set the tb_limit attribute of CompositeError to adjust this.

Prompted by [this SO question](http://stackoverflow.com/questions/14635935/silence-or-condence-ipython-parallel-exceptions).

IPython/parallel/error.py

@@ -232,6 +232,8 b' class TaskRejectError(KernelError):'
 
 class CompositeError(RemoteError):
     """Error for representing possibly multiple errors on engines"""
+    tb_limit = 4 # limit on how many tracebacks to draw
+    
     def __init__(self, message, elist):
         Exception.__init__(self, *(message, elist))
         # Don't use pack_exception because it will conflict with the .message
@@ -256,22 +258,28 b' class CompositeError(RemoteError):'
 
     def __str__(self):
         s = str(self.msg)
-        for en, ev, etb, ei in self.elist:
+        for en, ev, etb, ei in self.elist[:self.tb_limit]:
             engine_str = self._get_engine_str(ei)
             s = s + '\n' + engine_str + en + ': ' + str(ev)
+        if len(self.elist) > self.tb_limit:
+            s = s + '\n.... %i more exceptions ...' % (len(self.elist) - self.tb_limit)
         return s
 
     def __repr__(self):
-        return "CompositeError(%i)"%len(self.elist)
+        return "CompositeError(%i)" % len(self.elist)
     
     def render_traceback(self, excid=None):
         """render one or all of my tracebacks to a list of lines"""
         lines = []
         if excid is None:
-            for (en,ev,etb,ei) in self.elist:
+            for (en,ev,etb,ei) in self.elist[:self.tb_limit]:
                 lines.append(self._get_engine_str(ei))
                 lines.extend((etb or 'No traceback available').splitlines())
                 lines.append('')
+            if len(self.elist) > self.tb_limit:
+                lines.append(
+                    '... %i more exceptions ...' % (len(self.elist) - self.tb_limit)
+                )
         else:
             try:
                 en,ev,etb,ei = self.elist[excid]

IPython/parallel/tests/test_view.py

@@ -619,6 +619,33 b' class TestView(ClusterTestCase, ParametricTestCase):'
         self.assertEqual(io.stdout.count('by zero'), len(view), io.stdout)
         self.assertEqual(io.stdout.count(':execute'), len(view), io.stdout)
     
+    def test_compositeerror_truncate(self):
+        """Truncate CompositeErrors with many exceptions"""
+        view = self.client[:]
+        msg_ids = []
+        for i in range(10):
+            ar = view.execute("1/0")
+            msg_ids.extend(ar.msg_ids)
+        
+        ar = self.client.get_result(msg_ids)
+        try:
+            ar.get()
+        except error.CompositeError as e:
+            pass
+        else:
+            self.fail("Should have raised CompositeError")
+        
+        lines = e.render_traceback()
+        with capture_output() as io:
+            e.print_traceback()
+        
+        self.assertTrue("more exceptions" in lines[-1])
+        count = e.tb_limit
+        
+        self.assertEqual(io.stdout.count('ZeroDivisionError'), 2 * count, io.stdout)
+        self.assertEqual(io.stdout.count('by zero'), count, io.stdout)
+        self.assertEqual(io.stdout.count(':execute'), count, io.stdout)
+    
     @dec.skipif_not_matplotlib
     def test_magic_pylab(self):
         """%pylab works on engines"""