feat: add new error strings for images

Merged Paul Pestov requested to merge feature/#339-image-error-message into develop

closes #339 (closed)

This MR introduces a possibility to display more than one error message in image panel.

If no image key is given by API we display imageErrorMessageNotExists.

If image key is given by API but the dummy request to the image URL failed we display imageErrorMessageVPN as usual.

Edited by Paul Pestov

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