prettify default error page
renamed boss_html_error_template.dtl to boss_html_errors_template.dtl
new boss_html_error_template.dtl handles single error called from process_error/5

on a way corrected path problem with issue #316, _base_url has been forgotten
more files
added CSS to error page with "parse errors"
added comment to boss_web_controller.erl to better understand and distinguish between two error pages
Merge pull request #318 from danikp/pretty_error

Pretty error