Better error message for missing function used as pointer

[Per Östlund]

The following program:

partial function PF
end PF;

function f
  input PF pf;
end f;

function f2
algorithm
  _ := f(function invalidf(x = 2.0));
end f2;

gives the error message:

Error: Failed to elaborate expression: function invalidf(x = 2.0).

Saying that invalidf doesn't exist would be more helpful in this case.

[Per Östlund]

The new frontend says Variable invalidf not found in scope f2, which I think is good enough.