add Salesforce API error handling for salesforce_bulk gem;

Salesforce bulk API isn't so descriptive in error messages as simple
salesforce API.
These changes add ability to run failed query through
databasedotcom-rails gem and raise error with informative description
about what's going on.
error handling should be only in read-only mode;
Merge pull request #1 from Proffard/better-error-handling

add Salesforce API error handling for salesforce_bulk gem;