flymake: Highlighting erroneous lines

 
 5.8 Highlighting erroneous lines
 ================================
 
 Highlighting is implemented with overlays and happens in the process
 sentinel, after calling the cleanup function.  Two customizable faces
 are used: ‘flymake-errline’ and ‘flymake-warnline’.  Errors belonging
 outside the current buffer are considered to belong to line 1 of the
 current buffer.
 
    If the option ‘flymake-fringe-indicator-position’ is non-‘nil’,
 errors and warnings are also highlighted in the left or right fringe,
 using the bitmaps specified by ‘flymake-error-bitmap’ and
 ‘flymake-warning-bitmap’.