Support array of errors

[bajtos]

Quoting from strongloop/loopback#2123 (comment)

please take this into account while working on the production-grade error handler, to make sure it correctly handles errors that are not Error instances.

We need to support an array of errors here. See how strong-remoting does it now and implement similar logic here: https://github.com/strongloop/strong-remoting/pull/289/files

[loay]

I think it is already supporting the array of errors in JSON already.
I added 2 test cases.