Optional Verbose Errors

[ForbesLindesay]

This would provide TypeScript style error messages. Most of this should actually be pretty straightforward with the current implementation.

Primary description of the idea: https://github.com/pelotom/runtypes/issues/10

• request for multiple errors (https://github.com/pelotom/runtypes/issues/8) - I'd prefer not to do this. most validation libraries do offer this, and it's a source of massive complexity, which often makes it harder to make the individual errors good.
• request for structured error info (https://github.com/pelotom/runtypes/issues/39) - We could provide the originating Runtype, and the path as a structured array.