Provide more detailed error messages

Use the sqlite3_errmsg() API to retrieve more specific error
messages.

eg. Attempting to exec 'CREATE TABLE ExistingTableName (...)'
will now report 'table already exists: ExistingTableName' rather
than 'SQL logic error or missing database'

error_test.go

@@ -27,7 +27,7 @@ func TestFailures(t *testing.T) {
 	if err == nil {
 		_, err = db.Exec("drop table foo")
 	}
-	if err != ErrNotADB {
+	if err.Code != ErrNotADB {
 		t.Error("wrong error code for corrupted DB")
 	}
 	if err.Error() == "" {