Add context to error messages

[petdance]

It would be extremely helpful when debugging to (optionally of course) see the
line of HTML that tidy was complaining about.

line 187 column 30 - Warning: <input> isn't allowed in <table> elements
                        </tr><input type="hidden" name="showbooks" value="1">                    <tr>

line 205 column 37 - Warning: missing </font> before <p>
            <td>                    <font color="red"><p id="at-price-type-label">NYC Quote</p></font><br />

line 211 column 45 - Warning: missing </b> before <p>
                    </p>                    <b><p>Prices do not include cataloging or processing</p></b>            </td>

line 232 column 30 - Warning: <input> isn't allowed in <table> elements
                        </tr><input type="hidden" name="showbooks" value="1">                    <tr>

line 250 column 55 - Warning: <p> anchor "at-price-type-label" already defined
            <td>                    <font color="red"><p id="at-price-type-label">NYC Quote</p></font><br />

I have submitted this as a request in the underlying library. htacg/tidy-html5#706 If that's not something they want to add at the library level, then we can add it here in HTML::Tidy5.