fix: add http error responses (#494)

This features adds rfc7807 Problem detail responses when an error happens processing a request.

This will greatly improve the common issues  with "blank pages" and "404 pages" issues which should now properly tell the user what input was wrong (group that does not exist, container name that does not exist, etc.)

app/sessions/errors.go

@@ -0,0 +1,31 @@
+package sessions
+
+import (
+	"fmt"
+	"time"
+)
+
+type ErrGroupNotFound struct {
+	Group           string
+	AvailableGroups []string
+}
+
+func (g ErrGroupNotFound) Error() string {
+	return fmt.Sprintf("group %s not found", g.Group)
+}
+
+type ErrRequestBinding struct {
+	Err error
+}
+
+func (e ErrRequestBinding) Error() string {
+	return e.Err.Error()
+}
+
+type ErrTimeout struct {
+	Duration time.Duration
+}
+
+func (e ErrTimeout) Error() string {
+	return fmt.Sprintf("timeout after %s", e.Duration)
+}