Enable extended error codes.

[c14n]

This pull request adds an additional field ExtendedCode for extended SQLite3 error codes as defined in https://www.sqlite.org/c3ref/c_abort_rollback.html to the Error type, and defines constants for those error types.

This pull request is fully backwards compatible with code like:

if err.(sqlite3.Error).Code == sqlite3.ErrConstraint {
    // clumsily compare err.Error() against strings to figure out what the heck happened...
}

Additionally, the following is now possible:

if err.(sqlite3.Error).ExtendedCode == sqlite3.ErrConstraintUnique {
    // oh! my unique constraint failed?
}

Cheers!

[mattn]

Thank you