make else if generate helpful error when condition have an issue (#8274)

# Description

Fixes: #7575

# User-Facing Changes

Previously:
```
if❯ if false { "aaa" } else if $a { 'a' }
Error: nu::parser::parse_mismatch

  × Parse mismatch during operation.
   ╭─[entry #10:1:1]
 1 │ if false { "aaa" } else if $a { 'a' }
   ·                         ─┬
   ·                          ╰── expected block, closure or record
   ╰────

```

After:
```
❯ if false { "aaa" } else if $a { 'a' }
Error: nu::parser::variable_not_found

  × Variable not found.
   ╭─[entry #1:1:1]
 1 │ if false { "aaa" } else if $a { 'a' }
   ·                            ─┬
   ·                             ╰── variable not found
   ╰────

```


# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-parser/tests/test_parser.rs

@@ -1972,4 +1972,23 @@ mod input_types {
 
         assert!(matches!(err, ParseError::VariableNotFound(_)));
     }
+
+    #[test]
+    fn else_if_errors_correctly() {
+        let mut engine_state = EngineState::new();
+        add_declarations(&mut engine_state);
+
+        let mut working_set = StateWorkingSet::new(&engine_state);
+        let (_, err) = parse(
+            &mut working_set,
+            None,
+            b"if false { 'a' } else $foo { 'b' }",
+            true,
+            &[],
+        );
+
+        let err = err.unwrap();
+
+        assert!(matches!(err, ParseError::VariableNotFound(_)));
+    }
 }