Improve error messages when loading .riv files

[avivian]

If an incorrect path is provided to a .riv file, we return an errors stating: Bad header and Problem loading file: may be corrupt. This is caused by the runtime trying to parse the response from the server that is not a .riv file, probably some sort of text or html along with the 404 status code.

A large amount of issues raised on our web runtimes are related to this issue, as it causes confusion for our users.

We should give more meaningful, descriptive errors here to help our users debug their problems

[eballeste]

tbf, i did get caught up on this but quickly rebounded after I noticed the 404 right before the error