More user friendly error message

This is helpful as if tkinter is not installed the ImportError is caught
here and although the user has installed matplotlib the error message
says matplotlib is needed.
So this small change fixes this.

Author: Thalmann

mprof

@@ -253,7 +253,7 @@ def add_brackets(xloc, yloc, xshift=0, color="r", label=None, options=None):
     try:
         import pylab as pl
     except ImportError:
-        print("matplotlib is needed for plotting.")
+        print("matplotlib is needed for plotting and tkinter for visualisation.")
         sys.exit(1)
     height_ratio = 20.
     vsize = (pl.ylim()[1] - pl.ylim()[0]) / height_ratio