ENH : improve error invalid error message for subplot

Closes matplotlib#2098

If user passes in a not-three-digit integer, raise `ValueError`
instead of letting the error propagate up from much farther down
the stack

lib/matplotlib/figure.py

@@ -901,6 +901,10 @@ def add_subplot(self, *args, **kwargs):
 
         if len(args) == 1 and isinstance(args[0], int):
             args = tuple([int(c) for c in str(args[0])])
+            if len(args) != 3:
+                raise ValueError("Integer subplot specification must " +
+                                 "be a three digit number.  " +
+                                 "Not {n:d}".format(n=len(args)))
 
         if isinstance(args[0], SubplotBase):