invalid value for `tidb_server_memory_limit_gc_trigger` got different error info

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```sql
et global tidb_server_memory_limit_gc_trigger = '-1%';
set global tidb_server_memory_limit_gc_trigger = '20.2%';
set global tidb_server_memory_limit_gc_trigger = '100%';
set global tidb_server_memory_limit_gc_trigger = 'abc';
set global tidb_server_memory_limit_gc_trigger = '0';
```

### 2. What did you expect to see? (Required)
the error info is same to `Variable 'tidb_server_memory_limit_gc_trigger' can't be set to the value of ''`
### 3. What did you see instead (Required)
```sql
[15:02:57]MySQL root:(none)> set global tidb_server_memory_limit_gc_trigger = '-1%';
(1105, 'strconv.ParseFloat: parsing "-1%": invalid syntax')
[15:03:06]MySQL root:(none)> set global tidb_server_memory_limit_gc_trigger = '20.2%';
(1105, 'strconv.ParseFloat: parsing "20.2%": invalid syntax')
[15:03:32]MySQL root:(none)> set global tidb_server_memory_limit_gc_trigger = '100%';
(1105, 'strconv.ParseFloat: parsing "100%": invalid syntax')
[15:03:56]MySQL root:(none)> set global tidb_server_memory_limit_gc_trigger = 'abc';
(1105, 'strconv.ParseFloat: parsing "abc": invalid syntax')
[15:04:05]MySQL root:(none)> set global tidb_server_memory_limit_gc_trigger = '0';
(1231, "Variable 'tidb_server_memory_limit_gc_trigger' can't be set to the value of '0'")
```
### 4. What is your TiDB version? (Required)
```sql
[15:06:39]MySQL root:(none)> select tidb_version();
+-----------------------------------------------------------+
| tidb_version() |
+-----------------------------------------------------------+
| Release Version: v6.5.0 |
| Edition: Community |
| Git Commit Hash: 77776e17159e9c6faf6aeedf3085a7a2524d3205 |
| Git Branch: heads/refs/tags/v6.5.0 |
| UTC Build Time: 2022-12-05 11:12:00 |
| GoVersion: go1.19.3 |
| Race Enabled: false |
| TiKV Min Version: 6.2.0-alpha |
| Check Table Before Drop: false |
| Store: tikv |
+-----------------------------------------------------------+
```