Protect zmqshell against non-unicode safe exceptions.

Closes gh-1827

IPython/zmq/zmqshell.py

@@ -40,6 +40,7 b' from IPython.testing.skipdoctest import skip_doctest'
 from IPython.utils import io
 from IPython.utils.jsonutil import json_clean
 from IPython.utils.process import arg_split
+from IPython.utils import py3compat
 from IPython.utils.traitlets import Instance, Type, Dict, CBool, CBytes
 from IPython.utils.warn import warn, error
 from IPython.zmq.displayhook import ZMQShellDisplayHook, _encode_binary
@@ -436,6 +437,27 b' class KernelMagics(Magics):'
             error("Could not start qtconsole: %r" % e)
             return
 
+def safe_unicode(e):
+    """unicode(e) with various fallbacks. Used for exceptions, which may not be
+    safe to call unicode() on.
+    """
+    try:
+        return unicode(e)
+    except UnicodeError:
+        pass
+
+    try:
+        return py3compat.str_to_unicode(str(e))
+    except UnicodeError:
+        pass
+
+    try:
+        return py3compat.str_to_unicode(repr(e))
+    except UnicodeError:
+        pass
+
+    return u'Unrecoverably corrupt evalue'
+
 
 class ZMQInteractiveShell(InteractiveShell):
     """A subclass of InteractiveShell for ZMQ."""
@@ -514,7 +536,7 b' class ZMQInteractiveShell(InteractiveShell):'
         exc_content = {
             u'traceback' : stb,
             u'ename' : unicode(etype.__name__),
-            u'evalue' : unicode(evalue)
+            u'evalue' : safe_unicode(evalue)
         }
 
         dh = self.displayhook