more improvements in sql code

src/error.rs

@@ -0,0 +1,23 @@
+use failure::Fail;
+
+#[derive(Debug, Fail)]
+pub enum Error {
+    #[fail(display = "Sqlite Error: {:?}", _0)]
+    Sql(rusqlite::Error),
+    #[fail(display = "{:?}", _0)]
+    Failure(failure::Error),
+}
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+impl From<rusqlite::Error> for Error {
+    fn from(err: rusqlite::Error) -> Error {
+        Error::Sql(err)
+    }
+}
+
+impl From<failure::Error> for Error {
+    fn from(err: failure::Error) -> Error {
+        Error::Failure(err)
+    }
+}