Improve CheckError error messages

[martonvago]

Following Luke's suggestions, we should improve the error messages generated by jsonschema and exposed by the checks package.

So, in checks, we should do the following modifications to all errors before returning them:

• Type error message to: f"The '{field}' field needs to be a 'string'."
• Required error message to: f"The '{field}' field needs to have a value in it, it is required."

Some options:

1. In validation_errors_to_check_errors, have a custom function generate the error message based on the ValidationError.
2. Have CheckError itself generate the error message based on the validator and the name of the field when the error in constructed.

Make sure that CheckErrors raised in Sprout use these same error messages.

[martonvago]

What do we think of not including the field in the message? Because it is already in the json_path and errors are stringified like:

Error at `$.id` caused by `required`: The 'id' field needs to have a value in it, it is required.