config: Improve logging of errors

Currently, the config package logs errors itself, in addition to
returning them.

That is confusing and results in some duplication of logging.

This patch makes config just return errors, and adjusts the callers
to log them properly.

internal/config/config.go

@@ -5,6 +5,7 @@ package config
 //go:generate protoc --go_out=. --go_opt=paths=source_relative config.proto
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 
@@ -19,15 +20,12 @@ func Load(path string) (*Config, error) {
 
 	buf, err := ioutil.ReadFile(path)
 	if err != nil {
-		log.Errorf("Failed to read config at %q", path)
-		log.Errorf("  (%v)", err)
-		return nil, err
+		return nil, fmt.Errorf("failed to read config at %q: %v", path, err)
 	}
 
 	err = prototext.Unmarshal(buf, c)
 	if err != nil {
-		log.Errorf("Error parsing config: %v", err)
-		return nil, err
+		return nil, fmt.Errorf("parsing config: %v", err)
 	}
 
 	// Fill in defaults for anything that's missing.
@@ -35,8 +33,7 @@ func Load(path string) (*Config, error) {
 	if c.Hostname == "" {
 		c.Hostname, err = os.Hostname()
 		if err != nil {
-			log.Errorf("Could not get hostname: %v", err)
-			return nil, err
+			return nil, fmt.Errorf("could not get hostname: %v", err)
 		}
 	}