DataSource.Error: Web.Contents failed to get contents from Salesforce.com

So after playing around with this for a while here's what I think - 

This is an issue that you can get around by using the choose columns step after bringing in the SF object. The reason this works is because of query folding in PowerBI - the query that PowerBI sends to SF after you include a remove/choose columns step is small enough to work.

However! If there is anything else in your query that would turn OFF query folding - (for me, this happened when I added a relative date filter), then this solution will no longer work, because now PowerBI is trying to query the full SF object, and then after that comes back apply a choose/remove columns. 

So - the workaround that worked for me, if available to you - put the base query in a PBI dataflow (SF connection and choose columns), then bring in the dataflow to PBI and apply any additional transformation to that. 

Here's a link to more infromation about query folding: https://docs.microsoft.com/en-us/power-query/power-query-folding

Hope this helps.