Return encode decode errors grpc

[nitinmohan87]

Without this change, goa returns a "fault" error from server even when the error was client side (e.g., goa request validation failed).

This PR makes sure that we return any validation errors during encode/decode as it is without changing them to grpc errors. I have also exported Goa's encode/decode error names so that it can be used in the design for mapping error name to a status code of choice.

NOTE: This is a breaking change because

• it changes the error name from "fault" to the actual error name that happened during encode/decode.
• it changes the grpc status code to "Unknown" instead of "InvalidArgument" unless the corresponding encode/decode error is mapped to a code of choice in the design (see example below)

Error(goa.InvalidLength)
GRPC(func() {
    Response(goa.InvalidLength, CodeInvalidArgument)
})

[raphael]

Thank you for the PR, looks great! Just one suggestion: could we also add the error constants to the DSL package so that they can be properly documented? The definitions could alias the ones in the pkg package. So in dsl/error.go there could be:

const (
	// The constants below make it possible for the service specific code to
	// return error names that are consistent with the names used by the
	// generated request and response payload validation code.
	// Usage:
	// [Provide meaningful example here]

	// InvalidFieldType is the error name for invalid field type errors.
	InvalidFieldType = pkg.InvalidFieldType
	// MissingField is the error name for missing field errors.
	MissingField = ...
)

[raphael]

Thank you!