Better error message for equation sections in functions needed

[Per Östlund]

Equation sections should give an error that says that equations sections are not allowed inside functions. Currently we say that equations failed to instantiate, i.e. for this model:

function f
  output Real x;
equation
  x = 42;
end f;

model m
  Real r = f();
end m;

we get the error message:

[model2.mo:4:3-4:8:writable] Error: Failed to instantiate equation x = 4;

[Martin Sjölund]

Fixed in trunk