Don't let invalid aliases crash IPython

e.g. `ipython --foo-bar=5` would raise a SyntaxError parsing the commmand-line, because `self.config.foo-bar=5` would be evaluated.

Syntax Errors on the rhs were already protected, but they could get into the lhs via the alias syntax.

closes gh-886

IPython/config/loader.py

@@ -502,7 +502,10 b' class KeyValueConfigLoader(CommandLineConfigLoader):'
                 if '.' not in lhs:
                     # probably a mistyped alias, but not technically illegal
                     warn.warn("Unrecognized alias: '%s', it will probably have no effect."%lhs)
-                self._exec_config_str(lhs, rhs)
+                try:
+                    self._exec_config_str(lhs, rhs)
+                except Exception:
+                    raise ArgumentError("Invalid argument: '%s'" % raw)
 
             elif flag_pattern.match(raw):
                 if item in flags: