Better exception on load files issue

Currently when there is an issue opening file we don't know what file failed. This might be more of an upstream improvement though.

https://github.com/status-im/nim-serialization/blob/master/serialization.nim#L91

Note that for privacy purposes it might be needed to have a switch to keep the current behaviour or maybe put `REDACTED` to show that the file path is purposefully kept private.