checker: improve fn argument mismatch error

[StunxFS]

This PR makes improvements to the changes introduced in the commit 7514783.

Before:

main.v:4:12: error: expected 2 arguments, but got 3
        have (int literal, string, bool)
        want (int, string)

    2 | 
    3 | fn main() {
    4 |     x(1, "2", true)
      |               ~~~~
    5 | }

After:

main.v:4:12: error: expected 2 arguments, but got 3
    2 | 
    3 | fn main() {
    4 |     x(1, "2", true)
      |               ~~~~
    5 | }
Details: have (int literal, string, bool)
         want (int, string)

[Delta456]

I like this change. Great work!