[Forwardport] Fix typo in Image::open exception message

[osrecio]

Original Pull Request

magento#15269

Description

Fixes a typo in the \Magento\Framework\Image::open exception message.

Previous message:

File {file} does not exists.

Corrected message:

File {file} does not exist.

Fixed Issues (if relevant)

N/A

Manual testing scenarios

1. Given that...

• There exists a page where an image is opened,
• but the actual image file does not exist,
• and the exception is not silently caught and ignored,

2. Expect to receive a properly-spelled exception message.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)