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

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.

Start at haskell-interactive-mode's handling of compiler diagnostics and inspect how warnings from -fdefer-type-errors and related flags are parsed. The work is done when those deferred type errors are marked as errors in the buffer without preventing features such as type at point or goto-defn from working.

Written by the indexing model from the issue text.