Better handling of errors when using `-fdefer-type-errors`

[TOTBWF]

I often use -fdefer-type-errors when using haskell-interactive-mode, as it lets me load files and get things like type at point, goto-defn, etc, even when there are type errors. However, it is a bit annoying that the type errors get reported as warnings in the buffer. It would be nice if we had some specific parsing for warnings generated by -fdefer-type-errors and friends, that marked them as errors in the buffer.