add FileWriteErrToErrMsg and FileReadErrToErrMsg

[Anton-4]

We currently have writeErrToStr and readErrToStr, but these just convert the tag name to a Str, they do not provide a high quility error message.

FileWriteErrToErrMsg and FileReadErrToErrMsg should convert each variant of a WriteErr or ReadErr to a high quality error message, for example with WasADirectory:

File write error:
    WasADirectory:
        I tried to write to:
            <path>
        but it was a directory, I can only write to a file.