config: construct "unset" error without using config::ConfigError

I also changed the error type from Config to User. They are failures of user
action, not configuration.

cli/src/config.rs

@@ -493,11 +493,11 @@ pub fn remove_config_value_from_file(
     let target_table = key_iter.try_fold(doc.as_table_mut(), |table, key| {
         table
             .get_mut(key)
-            .ok_or_else(|| ConfigError::NotFound(key.to_string()))
+            .ok_or_else(|| user_error(format!(r#""{key}" doesn't exist"#)))
             .and_then(|table| {
                 table
                     .as_table_mut()
-                    .ok_or_else(|| ConfigError::Message(format!(r#""{key}" is not a table"#)))
+                    .ok_or_else(|| user_error(format!(r#""{key}" is not a table"#)))
             })
     })?;
 
@@ -510,7 +510,7 @@ pub fn remove_config_value_from_file(
             entry.remove();
         }
         toml_edit::Entry::Vacant(_) => {
-            return Err(ConfigError::NotFound(key.to_string()).into());
+            return Err(user_error(format!(r#""{key}" doesn't exist"#)));
         }
     }
 

cli/tests/test_config_command.rs

@@ -613,10 +613,7 @@ fn test_config_unset_non_existent_key() {
     let repo_path = test_env.env_root().join("repo");
 
     let stderr = test_env.jj_cmd_failure(&repo_path, &["config", "unset", "--user", "nonexistent"]);
-    insta::assert_snapshot!(stderr, @r###"
-    Config error: configuration property "nonexistent" not found
-    For help, see https://martinvonz.github.io/jj/latest/config/.
-    "###);
+    insta::assert_snapshot!(stderr, @r#"Error: "nonexistent" doesn't exist"#);
 }
 
 #[test]
@@ -637,10 +634,7 @@ fn test_config_unset_inline_table_key() {
         &["config", "unset", "--user", "inline-table.foo"],
     );
 
-    insta::assert_snapshot!(stderr, @r###"
-    Config error: "inline-table" is not a table
-    For help, see https://martinvonz.github.io/jj/latest/config/.
-    "###);
+    insta::assert_snapshot!(stderr, @r#"Error: "inline-table" is not a table"#);
 }
 
 #[test]