Log an error if modbus Cover is not initialized correctly

[vzahradnik]

Proposed change

This PR adds an error message to the log, if user tries to initialize Cover
in an unsupported way, i.e., the discovery_info dictionary is None.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

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To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[probot-home-assistant]

Hey there @adamchengtkc, @janiversen, mind taking a look at this pull request as its been labeled with an integration (modbus) you are listed as a codeowner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[janiversen]

looks good

[vzahradnik]

@MartinHjelmare thanks for the review! I'll update the code here, as you suggested, and I'll do the same also in PR #42120 and #48558. This small PR was ported from my work there to maintain consistency with the components.

[janiversen]

@vzahradnik if you want, you can look in #49053 test_modbus.py I already changed mock(_logger) -> caplog

[vzahradnik]

@MartinHjelmare I updated the code as you requested. Please take a look at your convenience. Thanks!

[janiversen]

adding a flag expectFail is a good idea, please add it.

[vzahradnik]

adding a flag expectFail is a good idea, please add it.

Already did as a POC as I waited for your response. Please check the code.

[janiversen]

Looks good to me, thanks for not duplicating code.

[vzahradnik]

OK, let's wait for the test results. If they'll pass, this PR should address all issues pointed by @MartinHjelmare

[MartinHjelmare]

Looks good!