[error overlay] fix highlighted lines (#75940)
### What
We should treat each line's first character to determine the leading
sign, rather than treating the trimed line's first character as sign.
The bad case was the close tag `>` was treated as leading error sign
```
> <p // should be treated as errored line ✅
> // should not be treated as errored line ❌
```
Also align the attribute name for highlighted lines
