chore: emit more reasonable error message when using incomplete type …

[sssooonnnggg]

Emit more reasonable error message when using incomplete type in struct.
Example:

struct B
{
  a: A;
}
struct A
{
  a: int;
}

When parsing this schema, flatc will emit a confusing error message: "structs may contain only scalar or struct fields".
In fact, it's not allowed because struct A is an incomplete type when parsing struct B, we can't get A's size and other information at this point.