Add ui test for Error without Display impl

tests/ui/missing-display.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    First,
+    Second,
+}
+
+fn main() {}

tests/ui/missing-display.stderr

@@ -0,0 +1,9 @@
+error[E0277]: `Error` doesn't implement `std::fmt::Display`
+ --> $DIR/missing-display.rs:3:10
+  |
+3 | #[derive(Error, Debug)]
+  |          ^^^^^ `Error` cannot be formatted with the default formatter
+  |
+  = help: the trait `std::fmt::Display` is not implemented for `Error`
+  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
+  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)