step should not panic when SQLITE_BUSY encountered

this commit only logs the error, but can more refine grained
method should be used.

reference: https://github.com/jgallagher/rusqlite/blob/master/src/row.rs#L43

diesel/src/sqlite/connection/stmt.rs

@@ -123,6 +123,10 @@ impl Statement {
         match unsafe { ffi::sqlite3_step(self.inner_statement) } {
             ffi::SQLITE_DONE => None,
             ffi::SQLITE_ROW => Some(SqliteRow::new(self.inner_statement)),
+            ffi::SQLITE_BUSY => {
+                error!("SQLITE_BUSY encountered");
+                None
+            },
             error => panic!("{}", super::error_message(error)),
         }
     }