Expressions lacking all variables should give a better error message

[Patrick Farrell]

When a user creates an Expression but doesn't specify all variables:

from dolfin import *
a = Expression("a")

one gets an error message:

RuntimeError: expected a default value to all member variables in the Expression.

but it can be a lot of work to figure out which one is missing, especially if your expression is very complicated.

With the attached patch, the error message is much more useful:

RuntimeError: expected a default value to all member variables in the Expression: missing ['a']