Change error formatting to omit backtraces

The format specifier we use for printing errors returned by the main
program will include a backtrace in the output -- if one is available.
That's not necessarily a feature that we need: the causal chain of
errors should be sufficient and end users are unlikely to have use for a
backtrace.
This change adjusts the format specifier so that we only print the chain
of errors, albeit in a slightly different format:
- Previously:
  > Failed to generate OTP
  >
  > Caused by:
  >     Command error: The given slot is not programmed

- New:
  > Failed to generate OTP: Command error: The given slot is not programmed

CHANGELOG.md

@@ -3,6 +3,7 @@ Unreleased
 - Introduced extension support crate, `nitrocli-ext`
 - Introduced `otp-cache` core extension
 - Enabled usage of empty PWS slot fields
+- Changed error reporting format to make up only a single line
 - Added `NITROCLI_RESOLVED_USB_PATH` environment variable to be used by
   extensions
 - Allowed entering of `base32` encoded strings containing spaces

src/main.rs

@@ -233,7 +233,7 @@ fn evaluate_err(err: anyhow::Error, stderr: &mut dyn io::Write) -> i32 {
   if let Some(err) = err.root_cause().downcast_ref::<DirectExitError>() {
     err.0
   } else {
-    let _ = writeln!(stderr, "{:?}", err);
+    let _ = writeln!(stderr, "{:#}", err);
     1
   }
 }

src/tests/otp.rs

@@ -7,6 +7,18 @@ use super::*;
 
 use crate::args;
 
+#[test_device]
+fn error_format(model: nitrokey::Model) {
+  let (rc, out, err) = Nitrocli::new().model(model).run(&["otp", "get", "32"]);
+
+  assert_ne!(rc, 0);
+  assert_eq!(out, b"", "{}", String::from_utf8_lossy(&out));
+  assert_eq!(
+    err,
+    b"Failed to generate OTP: Library error: The given slot is invalid\n"
+  );
+}
+
 #[test_device]
 fn set_invalid_slot_raw(model: nitrokey::Model) {
   let (rc, out, err) = Nitrocli::new()