worker: use openapi spec and generated code

Write an openapi spec for the worker API and use `deepmap/oapi-codegen`
to generate scaffolding for the server-side using the `labstack/echo`
server.

Incidentally, echo by default returns the errors in the same format that
worker API always has:

    { "message": "..." }

The API itself is unchanged to make this change easier to understand. It
will be changed to better suit our needs in future commits.

vendor/github.com/pkg/errors/Makefile

@@ -0,0 +1,44 @@
+PKGS := github.com/pkg/errors
+SRCDIRS := $(shell go list -f '{{.Dir}}' $(PKGS))
+GO := go
+
+check: test vet gofmt misspell unconvert staticcheck ineffassign unparam
+
+test: 
+	$(GO) test $(PKGS)
+
+vet: | test
+	$(GO) vet $(PKGS)
+
+staticcheck:
+	$(GO) get honnef.co/go/tools/cmd/staticcheck
+	staticcheck -checks all $(PKGS)
+
+misspell:
+	$(GO) get github.com/client9/misspell/cmd/misspell
+	misspell \
+		-locale GB \
+		-error \
+		*.md *.go
+
+unconvert:
+	$(GO) get github.com/mdempsky/unconvert
+	unconvert -v $(PKGS)
+
+ineffassign:
+	$(GO) get github.com/gordonklaus/ineffassign
+	find $(SRCDIRS) -name '*.go' | xargs ineffassign
+
+pedantic: check errcheck
+
+unparam:
+	$(GO) get mvdan.cc/unparam
+	unparam ./...
+
+errcheck:
+	$(GO) get github.com/kisielk/errcheck
+	errcheck $(PKGS)
+
+gofmt:  
+	@echo Checking code is gofmted
+	@test -z "$(shell gofmt -s -l -d -e $(SRCDIRS) | tee /dev/stderr)"