Deprecate and remove dataPath from errors

[richfitz]

After #41 is merged, our verbose error format deviates from that of ajv, which has renamed dataPath to instancePath (see migration notes).

• add a ajv_errors class or similar to the errors element in the verbose output
• remove the dataPath element
• add a $.ajv_errors and [[.ajv_errors method that converts dataPath to instancePath with a warning