Show traceback, continuing to start kernel if pylab init fails

Error message goes directly over stderr (and traceback over stdout),
rather than as a pyerr message, which is associated with a particular
frontend, and thus ignored because startup actions were not requested
by anyone.

This better matches the terminal behavior, where `ipython --pylab` without matplotlib
installed shows a traceback, but continues to start.

In the qtconsole the traceback will come in below the banner, but in the notebook
the message will not arrive until after the first execution.

closes gh-990, closes gh-991.

IPython/zmq/ipkernel.py

@@ -743,9 +743,6 b' class IPKernelApp(KernelApp, InteractiveShellApp):'
 
     def init_kernel(self):
 
-        if self.pylab:
-            gui, backend = pylabtools.find_gui_and_backend(self.pylab)
-
         kernel = Kernel(config=self.config, session=self.session,
                                 shell_socket=self.shell_socket,
                                 iopub_socket=self.iopub_socket,
@@ -755,9 +752,31 b' class IPKernelApp(KernelApp, InteractiveShellApp):'
         )
         self.kernel = kernel
         kernel.record_ports(self.ports)
-
+        shell = kernel.shell
         if self.pylab:
-            kernel.shell.enable_pylab(gui, import_all=self.pylab_import_all)
+            try:
+                gui, backend = pylabtools.find_gui_and_backend(self.pylab)
+                shell.enable_pylab(gui, import_all=self.pylab_import_all)
+            except Exception:
+                self.log.error("Pylab initialization failed", exc_info=True)
+                # print exception straight to stdout, because normally 
+                # _showtraceback associates the reply with an execution, 
+                # which means frontends will never draw it, as this exception 
+                # is not associated with any execute request.
+                
+                # replace pyerr-sending traceback with stdout
+                _showtraceback = shell._showtraceback
+                def print_tb(etype, evalue, stb):
+                    print ("Error initializing pylab, pylab mode will not be active", file=io.stderr)
+                    print (shell.InteractiveTB.stb2text(stb), file=io.stdout)
+                shell._showtraceback = print_tb
+                
+                # send the traceback over stdout
+                shell.showtraceback(tb_offset=0)
+                
+                # restore proper _showtraceback method
+                shell._showtraceback = _showtraceback
+                
 
     def init_shell(self):
         self.shell = self.kernel.shell