Improve the error list

# User-visible changes

- Refactor the mode line handling in the error list buffer.  Don't set the mode line explicitly anymore, and instead just assign a fixed mode-line construct, and let `revert-buffer` take care of updating the mode line when necessary
- n/p move by error now, and show the location of the error in another window. See #452
- RET now jumps to the error at point
- Enable sorting of the error list by error level

# API changes

- Error levels have a `:severity` property now that denotes the numeric severity used for sorting
- Add `flycheck-error-level-<` to sort errors by error level