Enhance parse error messages

[akheron]

They're currently produced by io-ts's default reporter, and are very verbose and hard to decipher.

[cdimitroulas]

I guess it would be nice to have a couple of options for how the errors should be formatted. Some standard ones might be an array of strings for each key, or a single string for each key.

For example for a request body of type { name: string; age: number } you might get back a 400 response with { name?: string[]; age?: string[] } or { name?: string; age?: string } which are both nice and easy to handle as a client (and potentially even map directly to form fields in a UI)