Custom error

[tebeka]

Define way to create errors with custom message.

This change also automatically generates errors.go

[tebeka]

@pavius said:

I commented on the custom error thingie - asking for integration tests
Just add a few cases to the integration tests I wrote
Specifically here: https://github.com/nuclio/nuclio/blob/master/pkg/processor/runtime/golang/test/golang_test.go