WebComment: migrate to exception logging

• Migrates error logging to new exception-based technique. (addresses #580)
• Creates classes 'InvenioWebCommentError' for errors and 'InvenioWebCommentWarning' for warnings.
• Creates function 'tmpl_error()' to display errors.
• Deletes CFG_WEBCOMMENT_ERROR_MESSAGES and CFG_WEBCOMMENT_WARNING_MESSAGES vars.
• Deletes errors and warnings vars wherever it is needed.
• Modifies function 'tmpl_warnings'.
• Modifies type of elements in a list of warnings. This list contains tuples (warning_text, warning_color) where warning_color is 'green' or ''.
• Updates comments of functions if it is needed.
• Adds 'req' parameter in calls to 'register_exception' whenever it is available.
• Merge remark: amended check_int_arg_is_in_range due to WebSession WRT WebComment mismatch.