Reorganize errors object with structure for parameters

[JasonWeill]

Problem

Our current errors object used for the create-job form uses a mapping from input name to error (as a string). For parameters, this is cumbersome, because parameters are ordered but the errors object is not.

Proposed Solution

Revise the errors object to be keyed on the possible members of the model, including an array for parameters.

Revise the removeParameter function passed to the parameters picker to modify the errors object's parameters.

Additional context

This is a follow-up to #65.