Make filetest errors report full context (#2207)

* clif-util: do not convert `anyhow::Error`s into strings into `anyhow::Error`s

* filetests: Use the debug formatting of `anyhow::Error`s

This provides the full error context, not just the source error's message.

cranelift/filetests/src/runner.rs

@@ -48,7 +48,7 @@ impl Display for QueueEntry {
         let p = self.path.to_string_lossy();
         match self.state {
             State::Done(Ok(dur)) => write!(f, "{}.{:03} {}", dur.as_secs(), dur.subsec_millis(), p),
-            State::Done(Err(ref e)) => write!(f, "FAIL {}: {}", p, e),
+            State::Done(Err(ref e)) => write!(f, "FAIL {}: {:?}", p, e),
             _ => write!(f, "{}", p),
         }
     }