Add a Tabular and Errors to Structure

[b5]

Two new fields on Structure before we ship 0.1.0:

type Structure struct {
  // ... current fields omitted

  // Tabular indicates if this data is intended to be interpreted in a row/column format
 Tabular bool
  // The number of errors determined by validating this data against Schema
  Errors int
}

Note that Errors should always be encoded to json (as in, no omitempty), even if the number of errors is 0 (because 0 errors is good!).

Tabular is to indicate "this data is intended to be consumed as a table", which might apply to high-dimensional data, but almost always applies to certain formats like csv.

Steps to complete

• Ship jsonschema needs a method to report *all* errors in an instance jsonschema#15
• Ship replace dataset.Schema with jsonschema.Schema #43
• Ship Update detect package to emit jsonschema.RootSchema #46
• Add Fields, adjusting Assign and json coding funcs
• Adjust dsfs.CreateDataset to set the Errors field before writing
• add a clause in CreateDataset that sets Tabular = true if Structure.Format == "csv"
• ship a PR & Party

[b5]

Ok quick updates on this, after some thought, I think we can drop the Tabular field. I initially thought we'd need to rely on the tabular flag to manage the transition from tabular schemas to jsonschema, but we've found a way around that. If anything, Tabular may end up being a field of VizConfig. So two updates to this issue:

1. No more tabular field
2. Let's change the name of the new Errors field to ErrCount, which seems more accurate