Improve some jsondocck errors

src/tools/jsondocck/src/main.rs

@@ -205,7 +205,21 @@ fn check_command(command: Command, cache: &mut Cache) -> Result<(), CkError> {
                     let val = cache.get_value(&command.args[0])?;
                     let results = select(&val, &command.args[1]).unwrap();
                     let pat = string_to_value(&command.args[2], cache);
-                    results.contains(&pat.as_ref())
+                    let has = results.contains(&pat.as_ref());
+                    // Give better error for when @has check fails
+                    if !command.negated && !has {
+                        return Err(CkError::FailedCheck(
+                            format!(
+                                "{} matched to {:?} but didn't have {:?}",
+                                &command.args[1],
+                                results,
+                                pat.as_ref()
+                            ),
+                            command,
+                        ));
+                    } else {
+                        has
+                    }
                 }
                 _ => unreachable!(),
             }
@@ -233,7 +247,13 @@ fn check_command(command: Command, cache: &mut Cache) -> Result<(), CkError> {
             assert_eq!(command.args[1], "=", "Expected an `=`");
             let val = cache.get_value(&command.args[2])?;
             let results = select(&val, &command.args[3]).unwrap();
-            assert_eq!(results.len(), 1);
+            assert_eq!(
+                results.len(),
+                1,
+                "Didn't get 1 result for `{}`: got {:?}",
+                command.args[3],
+                results
+            );
             match results.len() {
                 0 => false,
                 1 => {