Validate errorDisplay as it can be disabled in options (fixes #3014)

[ntadej]

Description

All UI elements can be disabled in options for example

{
  'posterImage': false,
  'textTrackDisplay': false,
  'loadingSpinner': false,
  'bigPlayButton': false,
  'controlBar': false,
  'errorDisplay': false,
  'textTrackSettings': false
}

When errorDisplay is disabled it does not exist anymore. Thus its existence needs to be validated to avoid crashes. The one fixed occurs immediately after player is initialized.

Specific Changes proposed

Validate existence of this.errorDisplay. This fixes #3014.

Requirements Checklist

• Bug fixed
• Reviewed by Two Core Contributors

[gkatsev]

LGTM.

[brandonocasey]

LGTM