refactor(server): Convert existing log entries to structured format

This rewrites all existing log statements into the structured logrus
format. For consistency, all errors are always logged separately from
the primary message in a field called `error`.

Only the "info", "error" and "warn" severities are used.

tools/nixery/server/builder/archive.go

@@ -14,7 +14,6 @@ import (
 	"path/filepath"
 
 	"github.com/google/nixery/server/layers"
-	log "github.com/sirupsen/logrus"
 )
 
 // Create a new tarball from each of the paths in the list and write the tarball
@@ -33,7 +32,6 @@ func tarStorePaths(l *layers.Layer, w io.Writer) error {
 		return err
 	}
 
-	log.Printf("Created layer for '%s'\n", l.Hash())
 	return nil
 }