Fixe Issues/3735: Check for docker and docker daemon and fail before download

[JulianFlesch]

Closes #3735

Also adds a try...except around the container_fetcher initialization, to fail gracefully but with a meaningful error message.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

❌ Patch coverage is 46.66667% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.34%. Comparing base (4942d71) to head (95f62c2).

Files with missing lines	Patch %	Lines
nf_core/pipelines/download/download.py	33.33%	6 Missing ⚠️
nf_core/pipelines/download/docker.py	66.66%	2 Missing ⚠️

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[mashehu]

does the SingularityFetcher raise a similar error?

[mirpedrol]

It is better to raise a UserWarning or LookupError, we handle these in the commands. This should be updated in the nf_core/commands_pipelines.py, you can look at other commands to see an example.

Suggested change

	exit(1)
	raise UserWarning