Better failure mode for functions with a trailing semicolon

[croyzor]

Given the following function

f(Nat) -> { A -> B }
f(zero) = { .. }
f(succ(n)) = { f; g; h; }

the ; after h is a syntax error. The BRAT parser, having failed to parse line 3 as an expression, will try and parse it as a new declaration and complain that = should be replaced with -> or -o.

This is a very confusing error message for a very easy mistake!!