detect if returned error is of type InvokeResponse_Error

[smasher164]

Is your feature request related to a problem? Please describe.
We currently check the errorType field to determine the category of errors to which a message corresponds. However, the errorType field is set by reflecting on the error value's concrete type, which leads to an influx of errorString or other error types that show up. Since, Go doesn't support creating a named type (golang/go#16522), we need some way of setting a custom errorType value.

Describe the solution you'd like
The lambda handler already constructs an InvokeResponse_Error, after reflecting on the user-returned error. Just type-assert that the error's underlying concrete type is InvokeResponse_Error, and if so, use that. This will allow us to directly set its Type field, as necessary.

Describe alternatives you've considered
Currently, we work around this by predeclaring a whole host of "dummy" error wrapper types, like so:

type categoryA struct{ error }
func (e categoryA) Error() string   { return e.error.Error() }
type categoryB struct{ error }
func (e categoryB) Error() string   { return e.error.Error() }
type categoryC struct{ error }
func (e categoryC) Error() string   { return e.error.Error() }
type categoryD struct{ error }
func (e categoryD) Error() string   { return e.error.Error() }
...

[smasher164]

Note that this is also backwards compatible, since InvokeResponse_Error doesn't currently implement Error.