Start work on introducing machine-readable error codes.

This commit introduces a new type 'ErrNo', implementing the error
interface.  Constants for all sqlite3 error codes are provided
in the new source file "error.go".

error_test.go

@@ -0,0 +1,34 @@
+package sqlite3
+
+import (
+	"database/sql"
+	"io/ioutil"
+	"os"
+	"path"
+	"testing"
+)
+
+func TestFailures(t *testing.T) {
+	dirName, err := ioutil.TempDir("", "sqlite3")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dirName)
+
+	dbFileName := path.Join(dirName, "test.db")
+	f, err := os.Create(dbFileName)
+	if err != nil {
+		t.Error(err)
+	}
+	f.Write([]byte{1, 2, 3, 4, 5})
+	f.Close()
+
+	db, err := sql.Open("sqlite3", dbFileName)
+	if err == nil {
+		_, err = db.Exec("drop table foo")
+	}
+	if err != ErrNotADB {
+		t.Error("wrong error code for corrupted DB")
+	}
+	db.Close()
+}