Improve the look of error pages

• core/error_api.php
  (error_handler): make stuff prettier by using some formatting and hiding
  more information in the basic error view
  (error_proceed_url): new function that allows a page to provide a url to
  proceed to after the error page. If not proceed url is given before
  the error is triggered, a message telling the user to use the browser's
  back button is given.

• lang/strings_english.txt: remove 'ERROR: ' from the error strings

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2161 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9