Rename the config structs setDefaults to validate

This allows us to check for some required values and give an easy to respond
to error at startup rather than a lot of validation during run time.

config/homeserver.go

@@ -1,7 +1,41 @@
 package config
 
+import (
+	"errors"
+)
+
+var (
+	ErrHomeserverNoAddress = errors.New("no homeserver address specified")
+	ErrHomeserverNoDomain  = errors.New("no homeserver domain specified")
+)
+
 type homeserver struct {
 	Address        string `yaml:"address"`
 	Domain         string `yaml:"domain"`
 	StatusEndpoint string `yaml:"status_endpoint"`
 }
+
+func (h *homeserver) validate() error {
+	if h.Address == "" {
+		return ErrHomeserverNoAddress
+	}
+
+	if h.Domain == "" {
+		return ErrHomeserverNoDomain
+	}
+
+	return nil
+}
+
+func (h *homeserver) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	type rawHomeserver homeserver
+
+	raw := rawHomeserver{}
+	if err := unmarshal(&raw); err != nil {
+		return err
+	}
+
+	*h = homeserver(raw)
+
+	return h.validate()
+}