Merge pull request #1396 from takluyver/magic_tb_fix

In order for the %tb magic to work as expected, no other error can be raised by invoking it. Even if an error is caught immediately, it affects the 'last error' values in sys.

This cuts a try/except block out of ip.magic(). Happily, in this case there was a simpler way to do the same thing using `str.partition`

IPython/core/interactiveshell.py

@@ -2005,14 +2005,9 b' class InteractiveShell(SingletonConfigurable, Magic):'
         if next_input:
             self.set_next_input(next_input)
 
-        args = arg_s.split(' ',1)
-        magic_name = args[0]
+        magic_name, _, magic_args = arg_s.partition(' ')
         magic_name = magic_name.lstrip(prefilter.ESC_MAGIC)
 
-        try:
-            magic_args = args[1]
-        except IndexError:
-            magic_args = ''
         fn = getattr(self,'magic_'+magic_name,None)
         if fn is None:
             error("Magic function `%s` not found." % magic_name)