ENH: require iconpath returns existing file

It is an internal function which should return valid paths
for correct installation and code.
Make it easier to detect invalid icon paths.

Resolve #36.

Author: pavoljuhas

src/diffpy/pdfgui/gui/pdfguiglobals.py

@@ -59,6 +59,7 @@ def iconpath(iconfilename):
     Return string.
     """
     rv = os.path.join(APPDATADIR, 'icons', iconfilename)
+    assert os.path.isfile(rv), "icon file does not exist"
     return rv