pylab_not_importable: Catch all exceptions, not just RuntimeErrors.

This catches situations when `pylab` isn't installed or fails for
another reason like:

  ImportError: No module named backend_tkagg

Closes gh-2646.

IPython/lib/tests/test_irunner_pylab_magic.py

@@ -17,11 +17,11 b' from IPython.lib import irunner'
 from IPython.testing import decorators
 
 def pylab_not_importable():
-    """Test if importing pylab fails with RuntimeError (true when having no display)"""
+    """Test if importing pylab fails. (For example, when having no display)"""
     try:
         import pylab
         return False
-    except RuntimeError:
+    except:
         return True
 
 # Testing code begins