Error handling improvements (#71)

* Remove `emperror.dev/errors`, remove all `errors#Wrap` and `errors#WithStack` calls
* Improve logging in `server/backup.go`

server/errors.go

@@ -1,9 +1,11 @@
 package server
 
-import "emperror.dev/errors"
+import (
+	"github.com/pkg/errors"
+)
 
-var ErrIsRunning = errors.Sentinel("server is running")
-var ErrSuspended = errors.Sentinel("server is currently in a suspended state")
+var ErrIsRunning = errors.New("server is running")
+var ErrSuspended = errors.New("server is currently in a suspended state")
 
 type crashTooFrequent struct {
 }