ignore errors in shell.var_expand

If an error is raised in the formatter, leave it untransformed.  This means that var_expand("$foo") will return "$foo" if `foo` is undefined, but more importantly it will *not* raise when var_expand is used on literal code that doesn't expect to be expanded (e.g. %prun arguments).

Tests included.

IPython/core/interactiveshell.py

@@ -2675,7 +2675,12 b' class InteractiveShell(SingletonConfigurable, Magic):'
         ns = self.user_ns.copy()
         ns.update(sys._getframe(depth+1).f_locals)
         ns.pop('self', None)
-        return formatter.format(cmd, **ns)
+        try:
+            cmd = formatter.format(cmd, **ns)
+        except Exception:
+            # if formatter couldn't format, just let it go untransformed
+            pass
+        return cmd
 
     def mktempfile(self, data=None, prefix='ipython_edit_'):
         """Make a new tempfile and return its filename.

IPython/core/tests/test_interactiveshell.py

@@ -235,10 +235,24 b' class InteractiveShellTestCase(unittest.TestCase):'
         ip = get_ipython()
         ip.user_ns['f'] = u'Ca\xf1o'
         self.assertEqual(ip.var_expand(u'echo $f'), u'echo Ca\xf1o')
+        self.assertEqual(ip.var_expand(u'echo {f}'), u'echo Ca\xf1o')
+        self.assertEqual(ip.var_expand(u'echo {f[:-1]}'), u'echo Ca\xf1')
+        self.assertEqual(ip.var_expand(u'echo {1*2}'), u'echo 2')
 
         ip.user_ns['f'] = b'Ca\xc3\xb1o'
         # This should not raise any exception:
         ip.var_expand(u'echo $f')
+    
+    def test_bad_var_expand(self):
+        """var_expand on invalid formats shouldn't raise"""
+        ip = get_ipython()
+        
+        # SyntaxError
+        self.assertEqual(ip.var_expand(u"{'a':5}"), u"{'a':5}")
+        # NameError
+        self.assertEqual(ip.var_expand(u"{asdf}"), u"{asdf}")
+        # ZeroDivisionError
+        self.assertEqual(ip.var_expand(u"{1/0}"), u"{1/0}")
 
 
 class TestSafeExecfileNonAsciiPath(unittest.TestCase):