selfhost/typechecker: Output type mismatch error after generic type p…

[onsah]

…arameter number mismatch error

• Output Type mismatch after mismatched number of generic parameters for T error
• Extract Type mismatch error to a helper function

This gets us closer to pass constructor_with_different_generic_instance.jakt. But because currently unknown and void points to the same type the error message is not exactly as wanted. This change now additionally outputs

Error: Type mismatch: expected ‘B’, but got ‘[void]’
----- tests/typechecker/constructor_with_different_generic_instance.jakt:12:13
 11 | function main() { 
 12 |     mut a = A(b: []) 
                  ^- Type mismatch: expected ‘B’, but got ‘[void]’
 13 | } 
-----

The expected output is Type mismatch: expected ‘B’, but got ‘[unknown]’

[awesomekling]

This has conflicts.

[AtkinsSJ]

Closing this since it's had conflicts for 2 months with no further work.