Raise KeyError when specifying non existent column

[malclocke]

We have a setup where we occasionally try to bulk import from a database with a schema that is out of sync with the target. If the source database has columns that do not exist on the target a more meaningful error from bulk_import would help us debug.

This change causes the code to short circuit with a KeyError if a non existent column is specified in the initializer.

Error before:

Error:
BulkInsertWorkerTest#test_meaningul_error_is_presented_if_specifying_non_existant_columns:
NoMethodError: undefined method `name' for nil:NilClass
    .../bulk_insert/lib/bulk_insert/worker.rb:52:in `block in add'
    .../bulk_insert/lib/bulk_insert/worker.rb:51:in `map'
    .../bulk_insert/lib/bulk_insert/worker.rb:51:in `with_index'
    .../bulk_insert/lib/bulk_insert/worker.rb:51:in `add'
    .../bulk_insert/test/bulk_insert/worker_test.rb:280:in `block in <class:BulkInsertWorkerTest>'

Error after:

Error:
BulkInsertWorkerTest#test_meaningul_error_is_presented_if_specifying_non_existant_columns:
KeyError: key not found: "no_such_column"
    .../bulk_insert/lib/bulk_insert/worker.rb:28:in `fetch'
    .../bulk_insert/lib/bulk_insert/worker.rb:28:in `block in initialize'
    .../bulk_insert/lib/bulk_insert/worker.rb:28:in `map'
    .../bulk_insert/lib/bulk_insert/worker.rb:28:in `initialize'
    .../bulk_insert/test/bulk_insert/worker_test.rb:273:in `new'
    .../bulk_insert/test/bulk_insert/worker_test.rb:273:in `block in <class:BulkInsertWorkerTest>