added ability to set the number of logfile lines displayed on errors (number or 'ALL')
added api call for displayed_error_line_count
exception uses displayed_error_line_count directly, changed initilisation of displayed_error_line_count, checking raises ConfigError on errors, removed option to set lines by env
Merge pull request #7 from planthaber/error_lines

added ability to set the number of logfile lines displayed on errors (number or 'ALL')