Conversion from FromSqlError to Error

[kornelski]

Error needs column index, but FromSqlError doesn't have it. I've fudged it by putting obviously-wrong index value, and then hiding that fact in Display.

Ideally the Error should use Option<usize> for index (and type), but I assume it's better to avoid unnecessary backwards-compat break.

Fixes #555

[kornelski]

The test failure looks like a vcpkg installation issue, unlikely to be related to this change.

[gwenn]

Thank you.