feat: improve error printing

When using `Result::expect()`, (e.g. in the default `cot::main`)
errors are printed using `Debug` printer. This causes very ugly
errors to be produced. Instead, when the alternate mode is not being
used, just format the error nicely, along with the error source chain.

This roughly duplicates the `anyhow::Error` error printing logic:
https://docs.rs/anyhow/latest/anyhow/struct.Error.html#method.chain

Author: m4tx

cot-core/src/error/error_impl.rs

@@ -171,7 +171,26 @@ impl Error {
 
 impl Debug for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        Debug::fmt(&self.repr, f)
+        // If the alternate (`{:#?}`) formatting has been specified, print out the
+        // `Debug` formatting normally. If not, (which is the case when using
+        // `Result::unwrap()` or `Result::expect()`) pretty print the error.
+        if f.alternate() {
+            Debug::fmt(&self.repr, f)
+        } else {
+            Display::fmt(&self.repr.inner, f)?;
+
+            writeln!(f)?;
+            writeln!(f)?;
+            writeln!(f, "caused by:")?;
+            let mut source = self.source();
+            while let Some(e) = source {
+                writeln!(f, "    {e}")?;
+
+                source = e.source();
+            }
+
+            Ok(())
+        }
     }
 }