err_html_text
This directive is one way to customize Squid’s error messages. The error message
files contain printf-like tokens.
Squid dynamically replaces the tokens with appropriate values for each
error. If Squid encounters the token %L, it inserts the contents of this
directive. Note that none of the default error messages contain a
%L. Thus, to use this feature, you
must modify the default error files.