fix/test: add stderr files

tests/ui/err-01.stderr

@@ -0,0 +1,12 @@
+error[E0277]: `?` couldn't convert the error to `Kaboom`
+  --> tests/ui/err-01.rs:39:12
+   |
+38 | fn iffy() -> Result<(), Kaboom> {
+   |              ------------------ expected `Kaboom` because of this
+39 |     Err(Fatal)?
+   |               ^ the trait `From<Fatal>` is not implemented for `Kaboom`
+   |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+   = help: the following implementations were found:
+             <Kaboom as From<Bobo>>
+   = note: required because of the requirements on the impl of `FromResidual<Result<Infallible, Fatal>>` for `Result<(), Kaboom>`

tests/ui/err-02.stderr

@@ -0,0 +1,29 @@
+error[E0277]: `?` couldn't convert the error to `Kaboom`
+  --> tests/ui/err-02.rs:45:12
+   |
+44 | fn iffy() -> Result<(), Kaboom> {
+   |              ------------------ expected `Kaboom` because of this
+45 |     Err(Fatal)?
+   |               ^ the trait `From<Fatal>` is not implemented for `Kaboom`
+   |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+   = note: required because of the requirements on the impl of `FromResidual<Result<Infallible, Fatal>>` for `Result<(), Kaboom>`
+
+error[E0277]: `?` couldn't convert the error to `Kaboom`
+  --> tests/ui/err-02.rs:49:11
+   |
+48 | fn bobo() -> Result<(), Kaboom> {
+   |              ------------------ expected `Kaboom` because of this
+49 |     Err(Bobo)?
+   |              ^ the trait `From<Bobo>` is not implemented for `Kaboom`
+   |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+   = note: required because of the requirements on the impl of `FromResidual<Result<Infallible, Bobo>>` for `Result<(), Kaboom>`
+
+error[E0277]: the trait bound `Fatal: Fatality` is not satisfied
+  --> tests/ui/err-02.rs:34:1
+   |
+34 | #[fatality]
+   | ^^^^^^^^^^^ the trait `Fatality` is not implemented for `Fatal`
+   |
+   = note: this error originates in the attribute macro `fatality` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/err-03.stderr

@@ -0,0 +1,35 @@
+error[E0277]: `?` couldn't convert the error to `Kaboom`
+  --> tests/ui/err-03.rs:45:12
+   |
+44 | fn iffy() -> Result<(), Kaboom> {
+   |              ------------------ expected `Kaboom` because of this
+45 |     Err(Fatal)?
+   |               ^ the trait `From<Fatal>` is not implemented for `Kaboom`
+   |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+   = help: the following implementations were found:
+             <Kaboom as From<FatalKaboom>>
+             <Kaboom as From<JfyiKaboom>>
+   = note: required because of the requirements on the impl of `FromResidual<Result<Infallible, Fatal>>` for `Result<(), Kaboom>`
+
+error[E0277]: `?` couldn't convert the error to `Kaboom`
+  --> tests/ui/err-03.rs:49:11
+   |
+48 | fn bobo() -> Result<(), Kaboom> {
+   |              ------------------ expected `Kaboom` because of this
+49 |     Err(Bobo)?
+   |              ^ the trait `From<Bobo>` is not implemented for `Kaboom`
+   |
+   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
+   = help: the following implementations were found:
+             <Kaboom as From<FatalKaboom>>
+             <Kaboom as From<JfyiKaboom>>
+   = note: required because of the requirements on the impl of `FromResidual<Result<Infallible, Bobo>>` for `Result<(), Kaboom>`
+
+error[E0277]: the trait bound `Fatal: Fatality` is not satisfied
+  --> tests/ui/err-03.rs:34:1
+   |
+34 | #[fatality(splitable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^ the trait `Fatality` is not implemented for `Fatal`
+   |
+   = note: this error originates in the attribute macro `fatality` (in Nightly builds, run with -Z macro-backtrace for more info)