errornorm(...) fails with vector fields

If the dimensionality of a vector field is greater than that of the hosting mesh, errornorm(...) will needlessly bail out with

Error:   Unable to interpolate function into function space.
*** Reason:  Dimension 0 of function (3) does not match dimension 0 of function space (2).

MWE:

from dolfin import *

# Two-dimensional mesh
mesh = UnitSquareMesh(20, 20, 'crossed')

V = FunctionSpace(mesh, 'CG', 1)

# Three-dimensional vector field
W = MixedFunctionSpace([V, V, V])

f0 = Expression(('sin(x[1])', 'cos(x[0])', 'tan(x[0]*x[1])'))
f1 = Expression(('tan(x[1])', 'sin(x[0])', 'cos(x[0]*x[1])'))

u0 = project(f0, W)
u1 = project(f1, W)

# Error:   Unable to interpolate function into function space.
# *** Reason:  Dimension 0 of function (3) does not match dimension 0 of function space (2).
errornorm(u0, u1)

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