The ErrorList plugin highlights errors in-place in the text area with a colored underline. The color depends on the error type; genuine errors have a red underline, warnings have a yellow one.
Hovering the mouse over a line with an error highlight will show a tooltip with the error message.
Error highlighting can be disabled in the Error List pane of the Plugin Options dialog box. The colors of the underlines can also be changed in that same option pane.