refactor: untangle workspace creation from http logic (#19639)

Coder Tasks requires us to create a workspace, but we want to be able to
return a `codersdk.Task` instead of a `codersdk.Workspace`. This
requires untangling `createWorkspace` from directly writing to
`http.ResponseWriter`.

coderd/httpapi/httperror/responserror.go

@@ -1,8 +1,12 @@
 package httperror
 
 import (
+	"context"
 	"errors"
+	"fmt"
+	"net/http"
 
+	"github.com/coder/coder/v2/coderd/httpapi"
 	"github.com/coder/coder/v2/codersdk"
 )
 
@@ -17,3 +21,48 @@ func IsResponder(err error) (Responder, bool) {
 	}
 	return nil, false
 }
+
+func NewResponseError(status int, resp codersdk.Response) error {
+	return &responseError{
+		status:   status,
+		response: resp,
+	}
+}
+
+func WriteResponseError(ctx context.Context, rw http.ResponseWriter, err error) {
+	if responseErr, ok := IsResponder(err); ok {
+		code, resp := responseErr.Response()
+
+		httpapi.Write(ctx, rw, code, resp)
+		return
+	}
+
+	httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+		Message: "Internal server error",
+		Detail:  err.Error(),
+	})
+}
+
+type responseError struct {
+	status   int
+	response codersdk.Response
+}
+
+var (
+	_ error     = (*responseError)(nil)
+	_ Responder = (*responseError)(nil)
+)
+
+func (e *responseError) Error() string {
+	return fmt.Sprintf("%s: %s", e.response.Message, e.response.Detail)
+}
+
+func (e *responseError) Status() int {
+	return e.status
+}
+
+func (e *responseError) Response() (int, codersdk.Response) {
+	return e.status, e.response
+}
+
+var ErrResourceNotFound = NewResponseError(http.StatusNotFound, httpapi.ResourceNotFoundResponse)