Fix printing struct name mismatch error

[kamilkowalski]

Hammox uses the struct_name_type_mismatch error when it detects an incorrect struct being used compared to what was specified. As I understand, it is raised in two cases:

• when a map is passed where a struct is expected,
• when struct A is passed where struct B is expected.

The second case has an error associated, but there was no function formatting this error into a human-readable message.

This PR adds a missing test case and fixes the problem by formatting the message correctly.

[msz]

Good catch. Thanks for your work!