The currently_pulling_query option can cause broken SOQL queries

[jonathanstegall]

Describe the bug
Users have reported, in #343 with a possible reason for the issue, as well as in the support forum (here, here, and with a possible solution here (thanks to zumajoe in WP forum).

To Reproduce
As far as I can tell, it's like this:

1. Create a fieldmap that works properly.
2. Delete some fields from Salesforce
3. The currently_pulling_query_{object_name} value in wp_options is not deleted.
4. The SOQL query will be wrong and cause errors

Expected behavior
Either we should delete that value from wp_options, or we should validate the fields in the stored query before running it.

[jonathanstegall]

I think the way to fix this is to check for a 400 error when the plugin runs a SOQL query. If that happens, we should clear the currently_pulling_query_{object_name} value from wp_options. Then, the next time the plugin ran, it would start over and regenerate the SOQL.