Add explicit check for pyqt4 to be initialized

This is moving the error to happen earlier for users of the package
since mantidplot only has sip exports for qt4.

MantidPlot/mantidplot.py

@@ -20,6 +20,9 @@ from pymantidplot import *
 # and the old qtiplot stuff
 import pymantidplot.qtiplot
 
+# error early if PyQt4 cannot be used
+import PyQt4
+
 def load_ui(caller_filename, ui_relfilename, baseinstance=None):
     '''This is copied from mantidqt.utils.qt and should be deprecated as
     soon as possible.'''