Prevent crash from invalid code such as a bare 'return'.

More generally, catch any exceptions from code that may have parsed
into an AST tree but doesn't actually compile.

Closes gh-704.

IPython/core/interactiveshell.py

@@ -2352,6 +2352,7 b' class InteractiveShell(SingletonConfigurable, Magic):'
             
         exec_count = self.execution_count
         
+        try:
         for i, node in enumerate(to_run_exec):
             mod = ast.Module([node])
             code = self.compile(mod, cell_name, "exec")
@@ -2363,6 +2364,17 b' class InteractiveShell(SingletonConfigurable, Magic):'
             code = self.compile(mod, cell_name, "single")
             if self.run_code(code):
                 return True
+        except:
+            # It's possible to have exceptions raised here, typically by
+            # compilation of odd code (such as a naked 'return' outside a
+            # function) that did parse but isn't valid. Typically the exception
+            # is a SyntaxError, but it's safest just to catch anything and show
+            # the user a traceback.
+
+            # We do only one try/except outside the loop to minimize the impact
+            # on runtime, and also because if any node in the node list is
+            # broken, we should stop execution completely.
+            self.showtraceback()
 
         return False