Add section name in Required exception message

[canro91]

Now, Required exception message only shows the name of the value to parse. To ease troubleshooting, Required exception message should display the section name too.

[Test]
public void Parse_RequiredAndPatternNotFound_ThrowsExceptionIncludingSectionName()
{
    var sectionName = "Key";
    var p = new Dictionary<String, IList<IParse>>
    {
        {
            sectionName,
            new List<IParse>
            {
                new Required(new ParseFromRegex(key: "Value", pattern: new Regex(@"Value:\s*(\d+)")))
            }
        }
    };

    var lines = FromText(@"
This line doesn't match");

    var parser = new Parser(p);

    var e = Assert.Throws<ArgumentNullException>(() => parser.Parse(lines));
    StringAssert.Contains(sectionName, e.Message);
}