Allow TryNext to have an error message without it affecting the command chain.

Closes gh-211

IPython/core/error.py

@@ -38,11 +38,18 b' class TryNext(IPythonCoreError):'
     should be used to handle the operation.  If you pass arguments to the
     constructor those arguments will be used by the next hook instead of the
     original ones.
+    
+    A _msg argument will not be passed on, so it can be used as a displayable
+    error message.
     """
 
-    def __init__(self, *args, **kwargs):
+    def __init__(self, _msg="", *args, **kwargs):
         self.args = args
         self.kwargs = kwargs
+        self.msg = _msg
+    
+    def __str__(self):
+        return str(self.msg)
 
 class UsageError(IPythonCoreError):
     """Error in magic function arguments, etc.

IPython/lib/clipboard.py

@@ -17,7 +17,7 b' def win32_clipboard_get():'
     except ImportError:
         message = ("Getting text from the clipboard requires the pywin32 "
             "extensions: http://sourceforge.net/projects/pywin32/")
-        raise TryNext(message)
+        raise TryNext(_msg=message)
     win32clipboard.OpenClipboard()
     text = win32clipboard.GetClipboardData(win32clipboard.CF_TEXT)
     # FIXME: convert \r\n to \n?
@@ -46,7 +46,7 b' def tkinter_clipboard_get():'
     except ImportError:
         message = ("Getting text from the clipboard on this platform "
             "requires Tkinter.")
-        raise TryNext(message)
+        raise TryNext(_msg=message)
     root = Tkinter.Tk()
     root.withdraw()
     text = root.clipboard_get()