[BUG] tests/check_gir.rs fails to build

[phodina]

Bug description
Attempting to build also the tests results in error as the trait std::fmt::Display is not impleented.

Backtrace

Compiling gtk v0.15.5 (/tmp/guix-build-rust-gtk-0.15.5.drv-0/gtk-0.15.5)
error[E0277]: `Errors` doesn't implement `std::fmt::Display`
 --> tests/check_gir.rs:6:20
  |
6 |     println!("{}", res);
  |                    ^^^ `Errors` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `Errors`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
  = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.